Serena Williams Billie Jean King WTA Finals
Billie Jean King presents Serena Williams with the WTA Finals trophyReuters

Serena Williams destroyed Romania's Simona Halep 6-3, 6-0 on Sunday to win the WTA Finals championship in Singapore, proving once again why she remains the most formidable force in women's tennis.

The American avenged her "embarrassing" loss to Halep in the group stage of the season-ending tournament with a ruthless display, overpowering her pint-sized opponent at every opportunity.

Halep shocked Serena 6-0, 6-2 in the group stage of the elite event but was unable to repeat that performance as the American reminded everyone why she is the number one player in the world.

"I lost to her a couple of days ago so I knew she was capable of playing well," Serena said in a courtside interview. "She started well and I knew I had to play well to beat her and I did play well."

For Serena, it was her third successive win in the WTA Finals, second only in prestige to the four grand slams, and her fifth overall, tied for second with Steffi Graf.

Only Martina Navratilova, with eight titles, has won more, but with Serena showing no signs of slowing down at the age of 33, it is not inconceivable the American could overhaul her.

The odds were stacked against Serena winning in Singapore after she pulled out of two events in China because of a knee injury then lost to Halep in the group stages.

But, as she has shown time and time again throughout her illustrious career, Serena has a knack for raising her game when it matters, as Halep learnt the hard way at Singapore's National Indoor Stadium.

"It was an amazing week for me so I cannot be sad that I lost," said Halep. "Congratulations to Serena -- you are the best."

Just as she did in their previous encounter, Halep made a confident start, holding her first service game to love and putting Williams under early pressure.

She broke Serena's second service game to lead 2-1 but failed to consolidate her early advantage as the American responded by taking her game up a notch.

Serena thumped two forehands past Halep to set up her first break point then levelled the scores at 2-2 when the Romanian dumped a forehand into the net.

Staring down Halep with each point she won, a fired-up Williams quickly began to assert her authority, and broke her opponent again to lead 4-2 when Halep hit a backhand long.

A lapse in concentration cost Serena the next game as successive double-faults allowed Halep to break back but those mistakes only spurred her on and she proceeded to reel off the final eight games in a row.

She blasted successive aces, the first at 161 km/h then the next at 194 km/h, to clinch the opening set then showed no mercy in the second, out muscling her opponent with her powerful serves, ground strokes and overhead smashes.

Halep won only seven points in the entire second set, that lasted just 25 minutes, before Serena sealed her win with her 26th winner and raised her arms in triumph as she captured her 64th career title and just over $2 million in prize money.

"I just said to myself that I had nothing to lose...and once I relaxed, I started playing better," Serena said.

"I really didn't expect this. I just started training because I had such a bad knee in Beijing and I didn't even know if I would be here."