Serena Williams remains on course to win a third successive WTA Finals title after beating Caroline Wozniacki in a marathon semi-final in Singapore on Saturday.

The American world number one fought back from a set down and a break in the decisive third set to win 2-6 6-3 7-6 (8/6) in two hours and 13 minutes.

Williams, who beat Maria Sharapova to win the 2012 title and then overcame Li Na to clinch last year's crown, will meet Simona Halep in Sunday's final at the Singapore Indoor Stadium.

Wozniacki made a blistering start to the last-four encounter, holding serve twice after breaking Williams' opening service game to race into a 3-0 lead.

Williams regained her composure, but, although she held her next two service games, Wozniacki did too.

Williams' frustration boiled over in the seventh game when she smashed her racquet after missing a break-point chance and her Danish opponent poured salt in her good friend's wounds by then breaking to win the first set 6-2.

Force a decider

Wozniacki made a decent start to the second set and won her first two service games, but Williams drew on all her experience to break twice to win the set 6-3 and force a decider.

All the American's hard work had seemingly gone to waste when Wozniacki broke to take a 5-4 lead and serve for the match, but the Dane could not close it out and the decider went to tie-break when both players held the subsequent service games.

Williams sent down her 12th ace to set up match point and she duly sealed the win when Wozniacki had no answer for one of her trademark thunderous forehand ground strokes.

It was Williams' fourth win against Wozniacki this year, including the US Open final and matches in Montreal and Cincinnati which also went to three sets.

"I didn't give a massive fist pump. I really wanted to cry because Caro is so nice we get along and we do so much together. Yeah, it was really kind of difficult," Williams said.

"But, you know what I really love about our relationship is we really play each other on the court incredibly hard. So, so, so, so, so hard. And then we just try to put it aside out of the court.

"But I didn't feel great about it, to be honest. She's like my little baby sister - from a different mother and father, and different country."

In the second semi-final, Romania's Halep beat Radwanska 6-2, 6-2 and will now be looking for a second straight victory over Williams.

The world No 4 handed a 6-0, 6-2 beating to Williams in the group stage earlier this week.
(Sky Sports)