Second seed Jelena Jankovic outlasted Victoria Azarenka 6-4, 6-7, 6-2 to reach the semi-finals of the Los Angeles Classic on Friday.
Jankovic will face fellow-Serbian and third seed Ana Ivanovic, who beat Maria Kirilenko of Russia 6-4, 6-4.
Fourth seed Nadia Petrova also advanced with a 6-7, 6-4, 6-1 victory over Virginie Razzano of France.
Petrova will face the winner of the match between top seed Maria Sharapova and holder Elena Dementieva.
Jankovic, who has been battling a bad cold all week, was not on top form and spent much of the match complaining to the chair umpire, whom she thought had lost control of the contest.
"I thought she was a little confused," Jankovic said. "She'd tell the wrong score, there were so many stoppages and complete confusion."
However, the Serbian managed to play steady enough in the third set to subdue the 18-year-old Belarusian, who was powerful yet inconsistent.
"I wasn't on my game, didn't have my rhythm and she was hitting so many let cord winners in the second set and I was thinking, god, how am I going to pass through because nothing is going my way. I just wanted to win and get out of there."
Ivanovic survived an up and down contest against Kirilenko, going down 1-4 in the first set, before taking the next seven games and then struggling to close it out.
"I was defending too much and she used the opportunities I was giving her," Ivanovic said. "Then I decided to be more aggressive, move forward and hit heavy spin to keep her outside the court."
Ivanovic has a 3-1-lifetime edge over her Serbian rival, but Jankovic won their meeting last year in the 2006 Los Angeles quarter-finals.
"It's nice to see someone else from your country is doing well but it's motivating for me because she's ranked number three and I would like to get over her."
Jankovic downplayed the rivalry, saying she has her sights set higher.
"I want to be the number one player in the world," Jankovic said. "I'm not into playing a Serbian and that it's a big deal. I want to be number two and have a rivalry with number one. I don't want to look behind me."
After a slow start, Petrova wore down Razzano, who was treated for a neck injury early in the third set.
"I felt much more fitter than her," Petrova said.