California - Vera Zvonareva fended off Sabine Lisicki, earning a 6-3, 3-6, 6-4 win on her fifth match point on Friday to lead the top three seeds into the semi-finals of the WTA San Diego Open.
The top-seeded Russian will play resurgent Ana Ivanovic, who escaped from 1-4 down in the opening set to defeat China's fourth seed Peng Shuai 6-4, 6-2.
Ivanovic, the 2008 Roland Garros champion, has been re-booting her game with a new coach and trainer and is seeing her hard work pay off.
The Serb, a former world number one who is trying to improve on her 18th ranking, equalled her best showing of the season by reaching the final four.
Ivanovic spun in an ace for a match point and advanced as Peng, who had her left hip taped in the second set, returned wide after 77 minutes.
"It was my first night match," said Ivanovic. "I'm happy I managed to calm down and get the break back.
"Then I was back in the match, I'm very pleased.
"Zvonareva will be a tough opponent, she's just coming off a title. But it will be a good test for me to see where my game is and what I need to work on."
Zvonareva, twice a Grand Slam finalist in 2010, overcame six double-faults to triumph in two and a quarter hours.
The Russian, who won her 12th career trophy in Azerbaijan last month, claimed her eighth consecutive win to drive her record in 2011 to 28-6 on hardcourts.
Second-seeded Andrea Petkovic earned a win for Germany as she motored past US teenager Sloane Stephens 6-2, 6-1 in 66 minutes.
Petkovic next faces third-seeded Agnieszka Radwanska, who shrugged off a sore right shoulder to beat eighth-seeded Daniela Hantuchova of Slovakia 0-6, 6-4, 6-4.
Radwanska admitted she considered packing in the match after dropping the first set without winning a game, but pride prompted her to play on.
"You're playing Daniela, who is a great player. I could lose the match, but not 6-0, 6-0. I've been playing too well to lose so quickly," she said. "That pushed me to keep fighting."
Radwanska has been bothered by a nerve problem in her shoulder which has affected her serve. She was broken six times during the win.
"Playing two hours is not really helpful for my shoulder, but I'll try my best tomorrow," said the Pole. "I didn't have much service, but I was just fighting till the end."
Petkovic, who has gone from strength to strength here as she launches her build-up to the US Open, schooled the 131st-ranked Stevens with four breaks.
"It's very important when you play these young players to show who's the boss on the court. There will come another time and another place when she will be a top player," said the winner.
Petkovic is on track to break into the WTA Top 10 for the first time. She's projected to become the first German woman in the Top 10 in the world rankings since Anke Huber in October of 2000.