Amid chants of "Si-mo-na, Si-mo-na" and the frenetic waving of Romanian flags, Serena Williams tore up the script and steamrolled over Simona Halep to win the BNP Paribas WTA Finals Singapore singles title on Sunday night, thrashing her opponent 6-3, 6-0.

It was the 33-year-old's third consecutive title at the prestigious year-ender, the first time a player has achieved that feat since Monica Seles in 1990-92. She wins US$2.05million (S$2.6 million) in prize money.

Williams, already assured of the year-end No. 1 ranking, had resumed training only last Monday having been troubled by a left knee injury which led her to withdraw from the Beijing Open.

Despite a slow start off the blocks, she recovered quickly and silenced the clusters of Halep fans. The American said shortly after her win: "Simona was playing so well. I just told myself to relax and once I did, I started playing better and making my shots."

Williams was handed her worst defeat in 16 years just days ago by Halep at the WTA Finals, going down 0-6, 2-6 in their round-robin encounter.

"I knew she was capable of playing really well. I knew I had to play better if I wanted to win, and I definitely wanted to win."

With the win, Williams picked up the winner's cheque along with the Billie Jean King trophy, which she also lifted in 2001, 2009, 2012 and 2013.

"I didn't expect this. I just started training because I had such a knee in Beijing so I wasn't even sure if I would be able to play here," she said. "And now I've won the Billie Jean King trophy again so for me it's just really amazing, I'm so excited."

Her victory at a packed Singapore Indoor Stadium was watched by Emeritus Senior Minister Goh Chok Tong. On top of the trophy, she also received a bouquet of orchids named after her, which she described as "the most important".

It is believed to be the first time an orchid hybrid has been named after a sports personality. In the past, the honour was conferred mainly on visiting dignitaries such as the Duke and Duchess of Cambridge, or more recently, the King and Queen of Malaysia.

After the match, Halep said: “It was an amazing week for me so I cannot be sad that I lost."

“Congratulations to Serena – you are the best.”