Kim Clijsters is bottom of her group after suffering a second defeat at the WTA Tour Championships in Los Angeles.
The Belgian, hoping to end the year as world number one, lost 6-3 7-6 (7-4) to France's Amelie Mauresmo.
Clijsters must now win her last match against Elena Dementieva and hope the Russian beats Mauresmo if she is to advance to the semi-finals.
The 22-year-old US Open champion has to go one round better than Lindsay Davenport to claim the number one spot.
Clijsters, a two-time winner of the season-ending tournament, said: "I was not at 100% to play my best tennis.
"These last two days, I haven't been hitting the ball very cleanly or seeing the ball.
"It's a matter of playing a lot and being a bit tired."
Pierce upsets Clijsters in opener
In the other match in Black Group, Pierce beat Dementieva 6-2 6-3 to move within touching distance of the semi-finals.
Pierce won comfortably to go top of the group and clinch the 500th win of her professional career.
The 30-year-old is only the third woman currently playing on the tour - after Davenport and Conchita Martinez - to have recorded 500 career wins.
She said: "That's pretty cool - I'm proud of that."
The Frenchwoman added that her opening victory over Clijsters had stood her in good stead.
"That was a great thing to play Kim first," she said.
"After playing her, everybody else is going to be easier because Kim is quickest."
Davenport, the top seed, is top of the Green Group after beating Patty Schnyder 6-3 7-5.
The American briefly looked in trouble when she double-faulted to trail 5-4 in the second set, but Schnyder failed to capitalise and dropped her serve immediately afterwards.
Davenport then held for a 6-5 lead and closed out the win when her opponent netted a backhand on the second match point.
She said: "Both of us got a little sloppy in the second set. It felt really tiring. I felt like I had to win in two sets."