Jankovic is playing her first tournament since Wimbledon
Top seed Jelena Jankovic maintained her bid to take over as the new world number one with an impressive 7-5 6-4 win over Nadia Petrova in Los Angeles.
Jankovic came back from breaks down in both sets to defeat the Russian in the East West Bank Classic quarter-finals.
The 23-year-old Serb, who will leapfrog compatriot Ana Ivanovic in the rankings if she wins the event, will face Dinara Safina in the last four.
The Russian crushed Victoria Azarenka 6-3 6-1 in her quarter-final.
The other semi-final pits Italian Flavia Pennetta, a 7-6 3-6 6-1 winner over 14th seeded-Austrian Sybille Bammer, against unseeded American Bethanie Mattek, who beat China's Yuan Meng 6-2 7-5.
Petrova tossed her racket angrily into her chair after missing an easy forehand volley to lose the first set, and let a 3-1 lead slip in the second before being broken again to lose the match.
I hung in there and hit my shots when I needed to
But she paid tribute to the progress of Jankovic, playing her first tournament since injuring her knee at Wimbledon last month.
"She's really improved over the past year," Petrova said. "She's not just running on the baseline anymore, she's serving better and taking the ball earlier and coming forward."
Jankovic said her knee was holding up well and was pleased with her movement.
"I played the big points well and was able to come back," she added.
"I did quite well when it was the important times. I hung in there and hit my shots when I needed to."