Russia's Nikolay Davydenko beat Argentine Gaston Gaudio 6-3, 6-4 to earn a spot in the last four at the season-ending Masters Cup on Wednesday.
In the other group match, Chile's Fernando Gonzalez beat Argentine Mariano Puerta 6-3, 4-6, 6-0 in his first match as a replacement.
World number seven Davydenko frustrated Gaudio with his pin-point accuracy to win his second straight match at the $4.45 million event in Shanghai.
Gaudio, who won last year's French Open, slipped to 1-1 following his victory over countryman Mariano Puerta on Monday and must now win his final Gold Group match to advance.
Davydenko had thrashed Andre Agassi 6-4, 6-2 in the first match of his Masters Cup debut, after which the American pulled out of the tournament citing an ankle injury.
French Open champion Rafael Nadal withdrew shortly before Agassi to join tournament no-shows Andy Roddick, Marat Safin and Lleyton Hewitt on the sidelines.
Gonzalez, who was drafted in as cover for the injured Andre Agassi on Monday, had too much power for his South American rival as he sent Puerta packing from the $4.45 million Shanghai event.
The first Chilean to appear at the season-ending championships since Marcelo Rios in 1998, Gonzalez's win also assured Russian Nikolay Davydenko's place in the last four.
The world number seven earlier beat Argentina's Gaston Gaudio 6-3 6-4 to make it two wins out of two in the Gold Group.
Puerta himself was a replacement for French Open champion Rafael Nadal, who pulled out shortly before Agassi to join Andy Roddick, Marat Safin and Lleyton Hewitt on the sidelines.
Puerta lost his opener to compatriot Gaudio.
World number one Roger Federer, chasing a third straight Masters Cup title, became the first man to reach the semi-finals on Tuesday.