SINGAPORE (AFP) - Serena Williams said "Serena-style tennis" helped her turn the tables on Simona Halep after she avenged her big defeat to the Romanian by crushing her to win the WTA Finals on Sunday. Part of that "style" included a karaoke machine that she had installed in her hotel room to keep her occupied.

"I sung pretty much every night," she said.

The 33-year-old Williams attended the Mariah Carey concert on Friday with her good friend and fellow tennis superstar, Caroline Wozniacki, and before the show the karoake machine also came into play.

"Before the Mariah Carey concert I sung most of her songs just to get ready," Williams said. "I didn't want to disappoint her as I screamed the lyrics to 'Hero'."

It all added up to victory.

The world number one was broken twice in a see-sawing first set but once she got the measure of her opponent she ran away with it 6-3, 6-0, four days after going down 6-0, 6-2 to the Romanian.

Williams' victory in 69 minutes erased memories of her worst loss in 16 years and helped her end a difficult 2014 on a high, with what was her third straight year-end title and fifth overall.

"I was trying to stay positive and trying to just stay out there and play better. Like I made so many errors in our last game. I knew for a fact I couldn't make that many errors," she said. "I had to play more Serena-style tennis and just do what I do best: enforce myself. That's what I was trying to do."

Williams could have departed the tournament at the round robin stage but results went her way to ensure she not only reached the semi-finals, but also was confirmed as year-end world number one.

The Finals brought a positive end to an "up-and-down" season for the 18-time Grand Slam-winner, who won seven titles including the US Open but disappointed at other major tournaments and had injury problems.

"It's been a really difficult year for me. I don't think I've had such an up-and-down year. I started out well with Brisbane, then didn't do well in Australia, then I did well again in Miami," she said. "It was really up-and-down. I ended the year well. I had a couple injuries in the beginning of Asia, but I'm glad I came and I was able to end well. So I think that was important.

"It's not how you start, it's how you finish, right?" Williams collected the gleaming Billie Jean King Trophy, the US$2.05 million (S$2.6 million) winner's cheque and a specially cultivated "Serena orchid" for her win - and said she liked the orchid the best.

Not to mention the karaoke machine again, but an obvious choice for her to sing about doing it "Serena-style" would be the Frank Sinatra evergreen, My Way. Not surprisingly, a reporter asked it it was "my way or the highway" at this stage of her career.

Williams said that while she was a fan of My Way, she preferred the song to the sentiment.

"No. Really, that was Frank Sinatra that did it his way," she responded. "I do sing that song, but that's about it."