In the Red Group, Spain's Carlos Moya defeated world No. 1 Lleyton Hewitt of Australia, 6-4, 7-5, after French Open champion Albert Costa of Spain topped third-ranked Marat Safin of Russia, 3-6, 6-4, 6-3.
In between, Jiri Novak of the Czech Republic rallied past No. 2 Andre Agassi, 7-5, 6-1, in a Gold Group matchup.
Moya beat Hewitt for the fourth straight time this year and advanced to 5-2 lifetime against the Australian. With a 2-0 record in the Red Group, he advanced to the semifinals.
"I beat the No. 1 player in the world, No. 3 in straight sets, both of them," Moya said. "So this is a very nice feeling."
"He's playing unbelievable tennis at the moment, as he's shown numerous times during the year, not only on clay, but also on hard court now as well," Hewitt said. "So, you know, it's definitely not bad loss losing to Carlos Moya."
The 26-year-old Spaniard reached the semifinals in his previous two appearances in the ATP's season-ending event. He lost a five-set final to countryman Alex Corretja in 1998 and was a semifinalist in 1997.
After finishing out of the top 10 each of the last three years, Moya has returned to the form that saw him win the 1998 French Open by claiming four titles this season.
"I think my level right now is very high," Moya said. "I cannot compare how it was in 1998 because it was four or five years ago, but I have been working very hard."
Making his Tennis Masters Cup debut, Novak beat Agassi for the first time in five encounters. He had never taken a set from the American before winning five straight games to win the opening set Wednesday. The 27-year-old Czech claimed 11 of the last 12 games to move to the top of the Gold Group.
The two players were to have squared off in the final of the Tennis Masters Series in Madrid last month, but Novak defaulted the championship match because of a right groin injury sustained in the semifinals. It was Agassi's tour-leading fifth tournament victory of the year. Novak is the only player entered here without a title this season.
The losses by Hewitt and Agassi might affect the race to finish the year No. 1. With his round-robin victory over Costa on Tuesday, Hewitt took a 108-point lead over Agassi in the 2002 ATP Champions Race.
The 21-year-old Hewitt has led the race every week since July 8 after winning his first Wimbledon title. He has owned the No. 1 ranking on the ATP Entry System every week since Nov. 19, 2001 after winning last year's Tennis Masters Cup title in Sydney.
In order to become the oldest year-end No. 1, the 32-year-old Agassi must win the title, with Hewitt losing in the semifinals with at least one group defeat. Agassi has upcoming matches against Roger Federer and Juan Carlos Ferrero while Hewitt next plays Safin.
"I tried my best and I was beaten," Agassi said. "I don't know about the (ranking) numbers or anything but it is not an easy turnaround for me to play a disappointing match, to lose a match and then come back and play the next day.
"I have always felt that once I lose, I deserve to be out of the tournament, so it is a difficult situation for me, but I will just have to see it as another opportunity, as another chance."