Holder Venus Williams completed her circuitous route to the WTA Championships final, beating Serbia's Jelena Jankovic 5-7, 6-3, 6-4 on Saturday to set up yet another showdown with sister Serena.
Serena, who will end the year as world number one for the first time since 2002, got past Caroline Wozniacki after the Dane pulled out with a stomach injury when trailing 6-4 0-1.
"I wasn't playing great. I think it's hard to play well against a player who's injured and is so nice," Serena told reporters. "I'm going to have to really improve my game against Venus if I want to win."
The Williams sisters have already met across the net in the Qatari capital this week, Serena winning a three-set scrap in their Maroon Group clash which was their 22nd career duel.
There will be no Williams clean sweep on Sunday though after they lost 2-6, 6-4, 10-8 in the doubles semi-final to Nuria Llagostera Vives and Maria Jose Martinez Sanchez.
The Spanish duo will face world number one pairing Cara Black of Zimbabwe and American Liezel Huber on Sunday.
Venus seemed down and out after defeat by her younger sister followed an opening group loss against Elena Dementieva.
After beating Svetlana Kuznetsova, though, some complicated maths edged her through to the last four by virtue of sets won.
It was all a bit unconvincing again on Saturday as the seven times grand slam champion sprayed errors around the court and also needed treatment on her knee several times.
Gradually she turned things in her favour after Jankovic committed the cardinal sin of failing to finish off a Williams sister when the chance presented itself.
Venus, 29, is the first player to reach the final after losing two matches in the round robin stage since Frenchwoman Amelie Mauresmo in 2003.
"In the first set I couldn't feel the ball and couldn't seem to get a ball in," said Venus.
LAST TOURNAMENT
"But this is my last tournament of the year so I was giving 1,000 percent," added the American world number seven, who has played four three-set matches at the Khalifa Tennis Centre totalling nearly eight hours.
"It's wonderful to be playing Serena in the final. I'm so excited. It's the way I wanted to end the year."
Jankovic, who also enjoyed good fortune to qualify for the last four when Dinara Safina retired after two games of their White Group clash on Wednesday, broke serve at 5-5 in the first set and then held her own to go ahead.
Venus appeared to be struggling more in the stifling humidity and served two double faults at 2-2 in the second to trail 0-40 only for Jankovic to let her opponent off the hook with some wasteful errors.
The Serb then saved two break points in the next game but Venus seized her chance when a third opportunity arose.
Jankovic broke serve early in the deciding set but surrendered her own delivery three times in a row and, although she did hang on at 3-5, Venus clinched victory in the next game with a forehand that dropped off the net tape.
"It's disappointing. I had so many chances. I had a lot of break points in the second set, but then I let everything slip away," said Jankovic who has dropped to number eight in the rankings after being number one in January.
"That was my last match of the season."
Wozniacki, too, looks in need of a rest. The world number four, who suffered agonising cramps during her group match win over Vera Zvonareva on Thursday, led 2-0 against Serena but then fell away.
"I had a great year. I did all I could. I used all my strength, but today I just didn't have any more left," said the 19-year-old U.S. Open runner-up.
"What I've learned is I'm a huge fighter."
