Defending champion Venus advanced to her second straight Doha final after rallying for a 5-7, 6-3, 6-4 win. Wozniacki then had to quit with an apparent abdominal injury when trailing Serena 6-4, 0-1.

Wozniacki received medical treatment on her stomach muscles in the first set and decided to quit after breaking Williams to start the second. The 19-year-old Dane has been battling injuries all week and said Friday that her aching abdominal muscles were affecting her serve.

disappointed

After chasing down a drop shot and hitting a forehand winner for the break, Wozniacki pulled up and then told the chair umpire she couldn't continue. She exchanged hugs and handshakes with Serena, explaining her decision with an apologetic smile.

"She's been struggling throughout the entire tournament," Serena said. "She's just an incredible fighter to come out here today and put up an effort."

Venus trailed 0-2 in the decisive set against Jankovic but broke the former top-ranked Serb three times in a row en route to a 5-3 lead. The defending champion then saved a break point in the last game before sealing the win with a forehand that bounced in off the net cord.

"In the first set, I couldn't feel the ball," Venus said. "Just to come back ... to play so well against such a consistent player is great."

Both players finished with more unforced errors than winners - 49 to 43 for Williams and 34 to 17 for Jankovic. Williams' serve was equally inconsistent, mixing 10 aces with 11 double faults.

It was the ninth three-setter in 10 meetings between the two, with the head-to-head series now level at 5-5. Their last meeting was also in the semi-final stage of the season-ending Doha tournament last year.

Williams advanced to the semi-finals despite winning just one of her three round-robin matches, the first player to do so since Amelie Mauresmo in 2003. She only clinched a berth when Svetlana Kuznetsova beat Elena Dementieva in the last group round on Friday, but seems determined to make the most of her lucky break.

"This is my last tournament of the year, so I'm giving a thousand per cent," she said.