Defending champion Venus Williams and sister Serena will clash for the 23rd time in their careers on Sunday in the final of the 4.55 million dollar WTA Championships.

Venus, the world number seven, defeated Serbia's Jelena Jankovic 5-7, 6-3, 6-4 in her semi-final on Saturday while 2001 champion Serena, the new world number one, went through when Caroline Wozniacki was forced to retire.

Danish teenager Wozniacki, who came into the tournament carrying a left thigh strain, and who had struggled with cramps in the round-robin stage, was 4-6, 1-0 when a stomach strain forced to her to quit.

"Caroline is a great fighter and a new face on the tour," said Serena, who has now won all four matches here including a three-set group win over her sister.

"I wasn't playing great. I think it's hard to play well against a player who's injured and is so nice."

Serena and Venus, who were later defeated in the doubles semi-finals in three sets by Spain's Nuria Llagostera Vives and Maria Jose Martinez Sanchez, will be far from fresh.

Serena played her semi-final with a tightly-bandaged left thigh - as did Wozniakci - while Venus needed treatment on her sore left knee in the first set of her clash against Jankovic.

With fitness worries on her mind, Serena earlier on Saturday pulled out of next weekend's Fed Cup final between the US and Italy.

Wozniacki, a US Open semi-finalist, admitted it was a match too far.

"I felt pain in my stomach muscle. I also had problems serving. But today after a while, the serve went over and also when I was hitting just regular groundstrokes, I started feeling it," she said.

"So it was just too much for me to keep playing."

For the 29-year-old Venus, it's been a remarkable tournament.

She only squeezed into the last four having lost two of her three group games and arrived in the semi-finals having spent five hours more on court than her Serbian opponent.

She was also forced into a fourth successive three-setter.

"It's my last tournament of the year, so I am trying to give a thousand percent," said world number seven Williams.

"I wasn't feeling the ball very well in the first set, but I came back and had to make sure I played well against such a consistent player like Jelena."

Venus believes playing her sister, who took her Wimbledon title in the summer, is the perfect way to end the season.

"It's wonderful. I'm so excited. It's the way I wanted to end the year," said the American who dropped an early break, recovered to 3-3 in the opener before Jankovic broke in the 11th game to lay the foundation for the first set.

Williams grabbed the only break of the second set in the sixth game, but was quickly down 2-0 in the decider before levelling immediately.

Jankovic, who was defeated by Williams in the semi-finals in 2008, should have been the fresher of the two.

She made the last four when Dinara Safina limped away after just two games, before she defeated a clearly unfit Wozniacki.

But despite reclaiming the break in the sixth game, she handed it back to allow the tenacious Williams to lead 4-3.

The American went to match point in the 10th game and clinched the win courtesy of a fortunate net cord.

Williams's win took her career record with Jankovic to 5-5, with the last nine meetings all going to three sets.