Petra Kvitova edged Yanina Wickmayer 5-7, 6-3, 7-6 (3) on Friday to reach the semifinals of the Open Gaz de France.
Kvitova of the Czech Republic will next face American Bethanie Mattek-Sands, who upset sixth-seeded German Andrea Petkovic 7-6 (4), 2-6, 6-3.
Belgium's Kim Clijsters, who played Jelena Dokic of Australia later, can clinch the top ranking with a victory.
Kvitova double-faulted to lose the first set to Wickmayer. But the fourth-seeded lefty broke for a 4-3 lead in the second set when Wickmayer netted a backhand. Kvitova leveled at one set apiece on a double fault from the seventh-seeded Belgian.
"In the first set, I had a good first serve but the second (serve) was terrible," said Kvitova, who hit six aces and seven double faults in the opening set.
Wickmayer broke Kvitova in the fifth game of the final set and was two points away from winning the match but a string of forehand errors allowed Kvitova to rally at 5-5.
Kvitova jumped to a 4-1 lead in the tie-breaker and converted her second match point when an approach shot from Wickmayer sailed long.
Kvitova trains at the same club as compatriots Tomas Berdych and Lucie Safarova and has now won 14 of 15 matches this season.
"I'm very happy to start the season like this," Kvitova said. "I didn't expect this. I know it will be tough to keep this shape and these results."
The Wimbledon semifinalist does not want to put any pressure on herself.
"For the next tournaments, I just want to be healthy and play my game. I haven't got a goal like a number," she said. "After Wimbledon, I just thought that I have to beat everyone. It was not good for me mentally."
Unforced errors from Petkovic helped Mattek-Sands win five straight games and lead 5-1 in the final set. The unseeded American held off a late charge from Petkovic, converting her first match point when the German sent a backhand into the net.
"It was a high-level match all around. Either person could have won obviously," Mattek-Sands said. "The great players, you have to rush them. If they have time, they're going to hit winners, especially someone like Andrea. When she sets up, she rips the ball."
The sixth-seeded Petkovic missed her chances in the opening set. Mattek-Sands saved two set points at 5-3 before hitting a forehand winner to break back in the 10th game.
"I felt like from 5-3 to 6-all, she was playing unbelievable," Petkovic said about her opponent. "I got really nervous."
In the tie-breaker, the American cracked a forehand winner down the line at 4-4 and then benefited from two unforced errors by Petkovic to take the first set.
"She deserved to win the first set although I had a few chances," Petkovic said. "I think the two set points I had, she served very well.
"She played aggressively. I was just one or two steps away from each ball and was just missing a lot."
Petkovic broke the American three times to win the second set before Mattek-Sands took control in the decider.
