Defending champion Venus Williams reached the final of the 4.55 million dollar WTA Championships on Saturday to set up a possible title match-up with sister Serena.

Williams, the world number seven, defeated Serbia's Jelena Jankovic 5-7, 6-3, 6-4 and will tackle either Serena or Danish teenager Caroline Wozniacki in Sunday's championship match.

It has been a remarkable week for the 29-year-old American.

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.

At one stage in the opening set, it looked as if the exertions had caught up with her as she needed treatment on her left knee having slipped to an early break down in the first set.

"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."

The American 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 here in 2008, should have been the fresher of the two women having made the semi-finals 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 then 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 which flopped over on to Jankovic's side of the net.