Second seed Caroline Wozniacki was pushed to a third-set decider against China's Zheng Jie before booking her place in the semi-finals of the BNP Paribas Open in Indian Wells.
The Dane was pushed all the way by her 18th-seeded opponent and was forced to fend off four break points in the final set before prevailing 6-4 4-6 6-1.
Conversion of break-point chances proved the difference as Wozniacki broke three times in the final set on her way to a first win in three matches against Zheng.
"It was a long match. It was really tough," Wozniacki said.
"She played really well and got a lot of balls back that I thought I had won already. I'm really pleased about my game today."
Next up for Wozniacki is a last-four encounter with Poland's Agnieszka Radwanska.
"She's a good player. She's getting a lot of balls back. She's thinking. She's smart on the court. But she's a good friend," Wozniacki said of Radwanska.
"Tomorrow I think we're just going to relax, have fun, and then we have to see what's gonna happen the day after."
Fifth seeded Radwanska booked her place in the semis after claiming a 6-4 6-3 win over fourth seed Elena Dementieva.
"I'm very happy that I am in this condition always playing two sets," said Radwanska.
"Of course, it's always better to win in two sets, but it's always hard, especially here."
"It's tough, especially against Dementieva, who is also very tough to play against. I'm just happy about my performance this week."
Dementieva, who hit 54 unforced errors in the match, added: "I didn't play the way I should be to beat her."
"I was playing her game, the way she feels pretty comfortable with the long rallies."
"I feel I was not aggressive with my footwork, was not aggressive with my shots today, so just way too many mistakes from me."