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Marcelo Andrés Ríos Mayorga (; born 26 December 1975) is a Chilean former professional tennis player. In 1998, he was ranked as the world No. 1 in men's singles by the Association of Tennis Professionals (ATP), becoming the first such Latin American. Ríos won 18 ATP Tour-level singles titles, including five Masters events, and was the runner-up at the 1998 Australian Open. He is the only man to have been world No. 1 in the ATP singles rankings without ever winning a major singles tournament.

Ríos was the first player to win all three clay court Masters tournaments (Monte Carlo, Rome, and Hamburg) since the format began in 1990. He was also the third man (after Michael Chang and Pete Sampras) to complete the Sunshine Double (winning Indian Wells and Miami Masters in the same year), which he achieved in 1998. At , Ríos is the shortest man to hold the number 1 ranking. He also held the top ranking in juniors. Ríos retired from the sport in July 2004 due to a long-term back injury.

He played his last ATP Tour-level tournament while only 27 years old at the 2003 French Open.