Serena Williams moved into her fourth consecutive final at the WTA Finals with a thrilling 2-6, 6-3, 7-6 (6) victory over Caroline Wozniacki, who heroically saved three match points before eventually surrendering to her good friend.

The 18-time Grand Slam-winner will play Simona Halep in Sunday's final — four days after her humiliating 6-0, 6-2 loss to the Romanian.

Just like in her opening match against Maria Sharapova, the stadium lights went crazy for a brief moment, but that didn’t stop Wozniacki from opening up a 3-0 lead to start the match.

The Dane maintained her lead towards the tail-end of the set and when she held for 5-2, Williams frustratingly destroyed her own racquet, smashing it numerous times on the ground.

Williams went down 0-40 the next game when Wozniacki punished her with a return winner. The American saved the first set point but faltered on the second, sending a crosscourt forehand wide to surrender the opening set.

Williams got her first set point with a return winner she had been trying to pull off from the start of the match. And the 33-year-old took the set when Wozniacki overcooked her second serve to double fault.

Wozniacki had a chance to break in the next game, benefitting from a couple of errors from Williams but the 24-year-old missed her first backhand in a long stretch of time to see her opportunity slip away.

The 12th game witnessed the point of the tournament so far. Facing a match point, Wozniacki ran down a drop shot, which sparked a series of drop shots and volleys ending with a winner at the net from the No8 seed, who held serve to force a tiebreak.

Wozniacki raced to 4-1 lead in the breaker but Williams took four points in a row to make it 5-4. The world No1 got two more match points but another volley winner from her opponent followed by a netted forehand error from herself kept Wozniacki’s hopes alive.

But Williams sealed the deal two points later, winning on her fourth match point when a Wozniacki forehand flew wide.

It was Williams’ 10th victory in 11 meetings with Wozniacki and her 25th win in her last 28 matches post-Wimbledon.

Halep continued her brilliant run in Singapore with a 6-2, 6-2 dismantling of Poland's Agnieszka Radwanska as the Singapore tournament's surprise package made it to the final on debut.

The diminutive world number four, backed by a phalanx of noisy Romanian fans, belted 26 winners against just four conceded as she broke Radwanska five times to win in 68 minutes.