The Chilean, fighting back to form after a four-month absence with a back injury, grew in confidence as the match progressed in hot conditions. The two clay-court specialists broke each other's serves early in the first set before Rios, ranked 13th in the world, began to get the upper hand.
In the quarter-final featuring two Spaniards, the 1996 champion Albert Costa, seeded fourth, overcame the sixth-seeded Felix Mantilla 6-2, 6- 4. The Frenchman Jerome Golmard defeated Karim Alami of Morocco 6-4, 7-6. Costa and Golmard will meet in today's semi-finals.
In the women's event the unseeded Hungarian, Rita Kuti Kis, continued her progress with a 6-3, 6-0 win over Anca Barna of Germany. The Estoril Open, a combined ATP and WTA event, is the traditional start to the European clay-court season.
Anna Kournikova registered a surprisingly easy 6-4 6-1 victory over the top-seeded world No 2, Lindsay Davenport, on Thursday night to reach the quarter-finals of the Bausch & Lomb Championships at Amelia Island, Florida.
It took just 54 minutes for the Kournikova, the world No 16, to dispose of the 22-year-old American. "I was just happy with the win," she said. "When you beat the No 2 in the world, you should be happy."
Combined with her appearance in the final of the Family Circle Cup last week, the Russian has a chance to move up as many as 11 places in the rankings by winning a title here. Thursday's victory was her most impressive since defeating Martina Hingis a year ago in Berlin. "This was important for me because I haven't beaten a top, top player in some time," she agreed. "So, this is very good. It gives me a lot of confidence. I need this."
Kournikova's next match will be against the No 6 seed, Patty Schnyder of Switzerland. The defending champion, Mary Pierce of France, who is the fourth seed, overcame her compatriot, Nathalie Dechy, 2-6, 6-4, 6- 3.