The top eight women in world tennis have limped to the season-ending WTA Finals in Singapore, with creaking knees and poor form casting a shadow over the WTA's latest big tournament push in Asia this week.

World No 1 Serena Williams headlines the event but there are concerns about her fitness after the 33-year-old American quit the China Open in Beijing earlier this month because of knee pain.

Williams went to Europe for tests and was one of the last to arrive in Singapore but said she was good to go as she bids for a third consecutive WTA Finals title and fifth overall.

"I'm ready. I really can't wait for my match. I wish I had a match tomorrow," the 18-time Grand Slam singles champion said after Saturday's draw.

Fit or not, her appearance at least will lower stress levels at the WTA, still reeling from the retirement of Chinese trailblazer and its biggest marketing tool Li Na weeks before the Singapore event.

Such is the power of Asia's first Grand Slam singles champion Li, the WTA has ensured she will be front and center to promote its latest tournament in Asia as the region continues to stump up cash and interest in women's tennis.

Singapore is the 22nd Asia-Pacific stop on the WTA's 58-tournament 2014 calendar, with the health of the Tour looking stronger than ever despite the lack of a title sponsor.

The health of the players, though, is an issue after a lengthy year of demanding tennis and travel saw plenty of retirements and lengthy injuries.

Serbia's Ana Ivanovic quit the Wuhan tournament in China and then the Linz Open with thigh and hip injuries while Canadian Eugenie Bouchard also pulled out of the Austria event 10 days ago with a leg injury.

"I won't have ideal training probably but I'll give it my best," Wimbledon runner-up Bouchard said at the time.

The three injury concerns have been pooled in the Red Group along with the out-of-form Simona Halep for the event which starts on Monday, with the top two advancing from the group stage to take on the best from a more competitive White Group.

French Open champion Maria Sharapova and Wimbledon victor Petra Kvitova look the most likely to get out of the White Group having won the past two big tournaments in Beijing and Wuhan to add to their Grand Slam titles this year.

However, Dane Caroline Wozniacki threatens their last-four hopes after a strong run to grab the final berth for Singapore following runner-up showings at the US Open and in Tokyo.

Pole Agnieszka Radwanska is the fourth member of the group, although her form has headed south since securing the Montreal title in August.