Ríos turned professional in 1994, finishing 1997, 1998, and 1999 as a top 10 player. Ríos won a total of 18 top-level singles titles and one top-level doubles title during his career.
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Ríos began playing tennis at the age of 11 at the Sport Francés golf club in Vitacura (Greater Santiago), adjacent to his house.
As a junior, Ríos reached as high as No. 1 in singles and No. 141 in doubles.
Ríos reached the semifinals of the junior French Open in 1993 without dropping a set, where he was defeated by Roberto Carretero-Diaz in straight sets, and won the junior US Open in 1993 while only dropping one set during the entire tournament. He also won his first satellite tournament in Chile.
This was Ríos' first year being a professional player and he quickly began to acquire international fame after his participation at Roland Garros, where in the second round, at just 18 years of age, he faced Pete Sampras, fighting a hard battle eventually to lose 6–7(5–7), 6–7(4–7), 4–6. His left-handed ability, plus his novel long hair and backwards visor, drew the attention of the media. The same year he won his first Challenger in Dresden, Germany.
In May 1995, aged 19, Ríos won his first tournament title in Bologna defeating Marcelo Filippini of Uruguay 6–2, 6–4, and breaking into the world's top 50 for the first time. Then in June he won at Amsterdam in both singles (against Jan Siemerink, 6–4, 7–5, 6–4) and doubles (with Sjeng Schalken) and won the tournament in Kuala Lumpur against Mark Philippoussis 7–6, 6–2. He also reached the final of his home country's ATP tournament in Santiago. Ríos ended the year ranked No. 25 in the world.
1996: Top 10 debut
Ríos' achievements this year included excellent performances in the Masters Series (then called Super 9) tournaments. He reached the quarterfinals in Masters Series of Stuttgart and Rome, and the semifinals in Indian Wells, Monte Carlo, and Canada. In Sankt Pölten, Austria, he won his fourth career title by defeating the Spaniard Félix Mantilla 6–1, 6–4. Ríos again reached the final in Santiago, and also reached the finals in Barcelona and Scottsdale. For much of the year Ríos would be ranked in the top 10, becoming the first Chilean in history to do so. He finished the year ranked number 11.
1997: Impending dominance
In 1997 for the first time in Ríos' career he reached the quarterfinals of a Grand Slam tournament at the Australia Open and again at the US Open. He also won his first Masters title at Monte Carlo; after a first round bye, he beat Andrea Gaudenzi, Albert Costa, Carlos Moyá, , and, in the final, Àlex Corretja, 6–4, 6–3, 6–3. Two weeks later he lost in the final of the Rome Masters against the same Spaniard. Other successes for the year included the quarterfinals (again) in the Stuttgart Masters and the finals in Marseille, Boston, and (for the third time) in Santiago. Ríos had a very consistent 1997 season, being the only player to reach the fourth round or better on all Grand Slams. Ríos went as high as No. 6 during the year, and ended the year in the top 10 for the first time, being No. 10.
1998: World No. 1 in singles, first Grand Slam final
The year 1998 brought the peak of the career of Ríos, who reached the No. 1 spot in the world. He won the tournament (the first of the year) in Auckland, New Zealand, against Richard Fromberg, then reached the final of the Australian Open, beating Grant Stafford, Thomas Enqvist, Andrew Ilie, Lionel Roux, Alberto Berasategui and Nicolas Escudé before losing to Petr Korda in a lopsided 2–6, 2–6, 2–6 that lasted 1 hour and 25 minutes. The following months brought successes such as the title of the Super 9 (the current Masters Series) at Indian Wells, where he defeated British Greg Rusedski in the final.
The consummation came in the final at Key Biscayne Masters, Florida, under the guidance of his coach Larry Stefanki. After victories over Hendrik Dreekmann, Tommy Haas, and Goran Ivanišević, Ríos beat Thomas Enqvist in the quarterfinals and Tim Henman in the semis. In the final on March 29, Ríos defeated Andre Agassi 7–5, 6–3, 6–4. In Chile, thousands of people took to the streets to celebrate the triumph of the first Chilean, Latin and indeed Spanish speaking player to reach the sport's No. 1 ranking, grabbing the position from Pete Sampras (who had maintained 102 consecutive weeks at No. 1, and five years ending the season as the leader). In the days ahead, there was a crowded reception leading Ríos to then president Eduardo Frei Ruiz-Tagle on La Moneda Palace, with around 10,000 people cheering at the palace's surroundings. Ríos' No. 1 ranking lasted four weeks; he lost it after being unable to defend the title at Monte Carlo because of an injury suffered in the Davis Cup while defeating Hernán Gumy in Buenos Aires, Argentina. In May he reached the quarterfinals of the French Open losing to eventual champion Carlos Moyá.
In June, at Wimbledon, Ríos was upset at first round by Spaniard Francisco Clavet. On August 10, however, Ríos recovered the No. 1 spot for another two weeks. In September he lost at third round of the U.S Open to . During this season Marcelo also won the Rome Masters against Albert Costa by walkover in the final, Sankt Pölten beating Vincent Spadea, the Grand Slam Cup against Andre Agassi, and Singapore against Mark Woodforde. Furthermore, he reached the quarterfinals in the Stuttgart Masters and Paris Masters. Ríos in 1998 won seven titles, including three Masters Series titles, and reached the final of the Australian Open. On July 27 of that year, he reached the maximum number of points achieved throughout his career: 3719 (by the scoring system used prior to the year 2000). He ended the year ranked No. 2 behind Pete Sampras, who topped the world rankings for a sixth consecutive year.
1999: Continued success and beginning of injuries
Ríos maintained a high level throughout 1999, although his game was interrupted by repeated injuries and surgeries. This prevented him from defending the points achieved by reaching the final of the Australian Open the previous year, so he fell several places in the rankings. He reached the final of the Monte Carlo Masters, but after trailing 4–6, 1–2, he had to retire due to a new injury, handing the tournament to Gustavo Kuerten. Ríos subsequently won the Hamburg Masters in a match that lasted more than four hours against Argentine Mariano Zabaleta; two weeks later he became champion in Sankt Pölten for the second consecutive time against the same Argentine, who, this time, had to retire during the first set at 4–4. In October, he won at Singapore and reached the final in Beijing, losing to the Swede Magnus Norman. He also reached the quarterfinals at Roland Garros and the Stuttgart Masters. Despite many injuries he suffered and surgeries he underwent, Ríos would complete his third consecutive year as a top 10 player, at world No. 9.
2000: Persistent injuries
Since 2000 until the end of Ríos' career on the main tour he was not able to keep up his level of play to the standards he set in the 1990s, as it was marked by repeated and disabling injuries. He still won the tournament of Umag, Croatia beating the Argentine Mariano Puerta in the final. Ríos also reached the semifinals at the Hamburg Masters losing to Marat Safin. Ríos finished the year No. 37 in the world.
In 2001 Ríos won the first tournament of the year in Doha. However, his performance in the following tournaments was weaker, weakened by an ankle operation, which resulted him to drop out of the top 50 in the world for the first time since he was a teenager. In September Ríos won another title, this time in Hong Kong, defeating German Rainer Schüttler in the final. Ríos decided to return in October to play a Challenger tournament in Santiago and defeated Argentine Edgardo Massa in the final, in an effort to end his curse of not winning an ATP tournament at his home country. He also reached a doubles final in Scottsdale. Ríos ended the year as No. 39 in the world.
In early 2002 Ríos had some good results, but a back injury prevented him from continuing the season successfully. It was the same injury that he had had two operations with already, and finally prevented him from returning to a competitive level. His best results were the quarterfinals at the Australian Open, the semifinal at the Miami Masters, and the final in the Stockholm tournament playing the Thai Paradorn Srichaphan. Ríos would finish Top 25 in the world for the first time since 1999 at No. 24, but without managing to recover from injuries that beset him since late 1999.
2003: Long absence from tour and out of Top 100
In Viña del Mar tournament (formerly Santiago tournament) Ríos reached the final, losing to Spaniard David Sánchez. This was the fourth of the four finals he participated and lost in his home country. However, representing Chile together with Fernando González and Nicolás Massú, he won the World Team Cup in Düsseldorf. The same year he also won silver medals in singles and doubles with Adrián García in the 2003 Pan American Games. In May Ríos played his last ATP-level match, losing in the first round at Roland Garros to Mario Ančić after retiring at 1–6 0–1. In 2003 Ríos played very few tournaments, in most of which he had to withdraw. This resulted him ending the year as No. 105 in the world, his worst year-end ranking on the main tour yet.
2004: Retirement from main tour
In 2004, six years after claiming the World No. 1 ranking, and after a long absence from the tour, Ríos returned to competition with a victory at a Challenger Series tournament in Ecuador. He played his last competitive match in early April 2004 at a Challenger in San Luis Potosí, Mexico, where he retired in the round of 16.
Finally, on July 16, 2004, after years of constant injuries—and at just 28 years old—Rios announced his retirement from tennis during a press conference in Santiago. He organized a farewell tour across his home country, travelling through several cities, meeting with fans, offering tennis clinics, and playing friendly matches with international and local tennis players such as Petr Korda and Goran Ivanišević. The tour ended on December 22, 2004, at a soccer stadium in Santiago, where he played his final tennis match of his career on the main tour against Guillermo Coria of Argentina.
2018: Desire for a comeback
Following successful elbow surgery in November, 2018, Ríos announced a comeback as part of his desire to become the oldest ever winner of a challenger tournament. 
On December 21, 2018, Ríos defeated Nicolás Lapentti 6-4, 5-7, 11-9 in an exhibition in Chile. 
ATP Champions Tour
On March 29, 2006, Ríos, aged 30, debuted on the ATP Champions Tour, a tour for former tour players, having met the requirement of at least two years after retirement. At his first tournament on the tour in Doha, Qatar, he defeated Thomas Muster, Henri Leconte, Pat Cash, and Cédric Pioline to claim the title. The following week he repeated, this time winning the crown in Hong Kong, where he won the final against Muster. Ríos won six tournaments in a row, adding Algarve, Graz, Paris and Eindhoven to the above. His inclusion on the senior circuit caused mild controversy, as he was significantly younger than many of his fellow competitors. He ended the year as No. 1, winning a total of six tournaments and holding a winning streak of 25 matches, achieving the record of being the only player in history to be No. 1 in the world as a junior, professional and veteran.
Ríos did not take part in the Champions Tour in 2007.
Ríos actually intended to return to the main ATP Tour in February at the Viña del Mar tournament (Movistar Open), but he defaulted because of the same back injury that made him retire from the tour.
On March 30, 2007, Ríos played an exhibition match in the Movistar Arena against Andre Agassi, both as a way to commemorate the match where Ríos rose to world No. 1 and as a way of having the American play in Chile.
In 2008 Ríos came back to veteran's tour where he won the tournaments in Barcelona and Algarve. On June 22, 2008, he was defeated by Pete Sampras in the final of a seniors tournament in São Paulo, Brazil. Ríos ended the year as No. 3 in the veteran's world rankings.
On June 24, 2008, Ríos defeated Sampras in an exhibition match that commemorated the 10-year anniversary of having reached the No. 1 ranking in the world.