Justine Henin-Hardenne beat Nadia Petrova in straight sets to secure her place in the last four of the WTA Championships in Madrid.
Henin-Hardenne has beaten Hingis and Petrova in her two matches
Petrova led 2-0 in the first set and netted an easy volley at 40-30 that would have sealed a 3-0 lead.
Henin then broke twice in succession to take the set and a single break in the second secured a 6-4 6-4 victory.
"It's amazing," said Henin-Hardenne. "I sensed Nadia was tired so I just tried to take my opportunities when I could."
Amelie Mauresmo came from behind to beat Martina Hingis 3-6 6-1 6-4 and maintain her own chances of making the semis.
Yellow Group standings
1. Henin W2 L0 F 4 A1
2. Petrova W1 L2 F2 A3
3. Mauresmo W1 L1 F1 A2
4. Hingis W1 L2 F2 A4
The Frenchwoman now needs to beat Henin-Hardenne on Friday to progress.
In the Red Group, Kim Clijsters beat Svetlana Kuznetsova 6-1 6-1 - taking just 45 minutes to wrap up victory.
"I was hitting the ball really well early in the match and she made a lot of errors," said Clijsters.
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"I got the breaks early and felt more confident."
Mauresmo said: "The main thing is to have the win today - I'm still alive in the group.
"I'll have to win against Justine - I'll take my chance."
Red Group standings
1. Sharapova W2 L0 F4 A0
2. Kuznetsova W1 L1 F2 A2
3. Clijsters W1 L1 F2 A2
4. Dementieva W0 L2 F0 A4
Petrova blamed the late hour of the matches for tiredness that contributed to a second consecutive defeat and exit from the tournament.
She said: "I think the players are not happy with the scheduling.
"Matches are starting very late, at 10pm, and it's really difficult when you have to come out and play the next day.
"It would have been nice to have the matches earlier so that we could have time to recover. I finished playing at midnight and by the time I got back to the hotel it was almost 2am."