DOHA: The Williams sisters are heading for another title matchup, after Venus defeated Jelena Jankovic on Saturday and Caroline Wozniacki retired

Serena against Serena in the WTA Championships semi-finals.

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.

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."

Serena, who also secured the year-end No. 1 ranking this week, is returning to the final of the tournament for the first time since 2004. She was knocked out in the group stage the last two years, but was the only player to go undefeated through the round-robin rounds this time.

"I just came in here trying to do my best, and here I am," she said.

The Williams sisters are also playing in the four-team doubles event at the season-ending tournament, and took on Spanish duo Nuria Llagostera Vives and Maria Jose Martinez Sanchez in the semi-finals later Saturday.

Venus trailed 2-0 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.