DOHA, Qatar -- The Williams sisters will end their season with another title matchup.
Serena, who secured the year-end No. 1 ranking this week, is returning to the final of the WTA Tour's season-ending championships 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.
Serena won the event in 2001 and has been runner-up twice. On Sunday, Venus will defend her title from last year.
"It's wonderful, I'm so excited," Venus said. "It's the way I wanted to end my year."
Serena beat Venus in this year's Wimbledon final, the fourth time they played each other for that Grand Slam title.
The American sisters were back on the court about 30 minutes after Serena's win Saturday to compete in the four-team doubles event. They missed out on a second final, however, losing to Spaniards Nuria Llagostera Vives and Maria Jose Martinez Sanchez 2-6, 6-4, 10-8 after a champions tiebreaker.
Wozniacki received medical treatment on her stomach in the first set and quit after breaking Williams to start the second. The 19-year-old Dane has been battling injuries all week, and reiterated after the match that aching abdominal muscles were affecting her serve.
"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."
After chasing down a drop shot and hitting a forehand winner for the break, Wozniacki pulled up and told the chair umpire she couldn't continue. The U.S. Open finalist exchanged hugs and handshakes with Serena, explaining her decision with an apologetic smile.
"It was just too much for me to keep playing," Wozniacki said. "I used all my strength I had left."
Serena has also been playing with her left thigh strapped, and she pulled out of next weekend's Fed Cup final against Italy.
"I think I'm just going to be wiped out by the end of this," she said. "I'm literally giving everything I have."
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 on a 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."
This was the ninth three-setter in 10 meetings between the two, with the head-to-head series now 5-5. Their last meeting was also in the semifinals in Doha last year.
Jankovic's ranking has slipped from No. 1 to No. 8 this year, and she was the last player to qualify for Doha. After a short vacation, she will go to work on reversing that trend.
"That will be my goal, to get up there to the top again," Jankovic said. "I have the ability to do that."
Venus reached this year's semis despite winning just one of her three round-robin matches, the first player to do so since Amelie Mauresmo in 2003. She clinched a berth when Svetlana Kuznetsova beat Elena Dementieva in the last group round Friday.
"This is my last tournament of the year, so I'm giving a thousand percent," she said.