Top-ranked Caroline Wozniacki needed to win two tiebreakers to advance at the Family Circle Cup on Thursday.
The top-seeded Dane defeated Barbora Zahlavova Strycova 7-6 (6), 7-6 (9) in a quarterfinal match that lasted more than 21/2 hours. It looked nothing like the 56-minute easy victory Wozniacki enjoyed in Wednesday's opening match.
"I knew that it wasn't going to be easy. I've seen her before and played her last year here," Wozniacki said.
She will next play the sixth seed, Yanina Wickmayer of Belgium, for a spot in the semifinals.
Samantha Stosur, the second seed and defending champion from Australia, fell to Elena Vesnina of Russia 6-4, 6-1, while third-seeded Jelena Jankovic of Serbia defeated Chanelle Scheepers 6-2, 6-2.
Jankovic will next face Christina McHale, an 18-year-old who is the youngest player among the world's top 100 at No. 97. McHale defeated Daniela Hantuchova 7-6 (3), 6-1.
Shahar Peer, ranked 11th, missed out on the chance to become the first Israeli woman ranked in the top 10 when she lost to Julia Goerges of Germany, 6-2, 6-3.
Wozniacki appeared to be cruising after opening up a 4-1 lead in the first set against Zahlavova Strycova. Then the frustrated Czech talked to her coach and promised to play more aggressively. The move worked: She extended points and forced Wozniacki deep into corners to chase down shots.
Zahlavova Strycova held serve twice to force the tiebreaker. She fought back in that one, too. After trailing 5-1, she won five straight points to hold a set point of her own. But Zahlavova Strycova sent that chance long and netted the next two points to give Wozniacki the set.
Zahlavova Strycova took a 2-0 lead into the second set. This time, it was Wozniacki's turn to rally. She twice held serve with Zahlavova Strycova ahead to force a second tiebreaker.
Wozniacki moved in front 5-2, but Zahlavova Strycova fought off four match points. Zahlovova Strycova then had a chance to take the set leading 9-8, but sent the ball wide.
Moments later, she put up a defensive lob on a hard hit ball into the deep right corner and Wozniacki pounded it into the grandstand.
Zahlavova Strycova said she was proud of her performance.
"Against these players, you have to risk a lot and you have to be aggressive," she said.
Wozniacki reached the finals two years ago before losing to Sabine Lisicki. She retired with an ankle injury in the semifinals last year.
She said she's counting on more difficult matches ahead in the clay-court season.
"This is the first tournament on clay," she said. "You need to get used to the conditions, you need to get used to sliding. There will be longer rallies in clay, there's no doubt about it, and the matches will most likely be longer."