Venus Williams of the United States hits a return against France's Caroline Garcia during the WTA Finals tennis tournament in Singapore, October 28, 2017. AFP/Roslan Rahman

SINGAPORE: Ageless Venus Williams continued her remarkable renaissance with a three-set semi-final victory over Caroline Garcia to set up a mouthwatering WTA Finals decider with in-form Caroline Wozniacki.

Williams, 37, came from a set down to triumph 6-7 (7/3), 6-2, and 6-3 in two hours and 29 minutes at the Singapore Indoor Stadium. She will be aiming to end a nine-year WTA Finals title drought when she plays Wozniacki on Sunday.

Garcia, playing in her WTA Finals debut, looked gassed at times, having played a two hour and 19-minute marathon against Wozniacki on Friday.

Williams claimed the decisive break in the eighth game of the third set before her clutch serving saw her overcame three break points and claim a memorable victory.

Williams said she was able to come to terms with Garcia, an opponent she hadn't played since 2014.

"I think in the second and third (sets) I was able to figure it out a little more," she told reporters after the match. "I haven't played her in a long time. She's improved her game so much."

The American has a commanding 7-0 record over Wozniacki but they haven't clashed for two years.

"I haven't played her in ages," Williams said. "I have to gauge how it goes and try to figure out the solution."

Garcia admitted she battled fatigue as the match wore on.

"I was missing a little bit of energy in the second and third set," she said. "It was very intense this match. The match from yesterday was very difficult physically and mentally."

Williams looked in grave trouble when she faced two break points in the first game of the second set but she fought back to hold serve and then broke Garcia in the next game.

Williams' trademark serve and her powerful groundstrokes started dictating, as she stormed through the second set in just 41 minutes.

Oldest finalist

The seven-time grand slam winner will be the oldest finalist in WTA Finals history when she plays Wozniacki. Even though she had a stellar season reaching two grand slam finals, the world number five will be seeking her first title of the year.

Earlier, a tireless Wozniacki stormed into the final with a straight-sets victory over Karolina Pliskova, ending the Czech's dreams of finishing world number one.

The world number six prevailed in a rollercoaster match 7-6 (11/9), 6-3 in one hour and 57 minutes to reach the decider of the WTA Finals for the first time since 2010.

Wozniacki said she kept "fighting" through a marathon 82-minute first set, where she held off three set points in the 10th game and almost blew a 6-1 lead in the tieback.

"It went back and forth," she told reporters after the match.

"It was very important to win the first set. I think that broke her a little bit. I think it was important for both of us to get that first one."

Pliskova said she would not stew over missing out on the top ranking. "I'm absolutely fine with that," she said. "This is the last thing that I'm thinking about now."

Wozniacki seemingly had a stranglehold on the match with an early break in the second set only for Pliskova to storm back. But she had a decisive break in the eighth game and then served it out to book her spot in the final.

Pliskova needed to lift the trophy on Sunday to overtake world number one Simona Halep in the rankings.

Even though she didn't make the semis, Halep has now secured the year-end world number one ranking.