SINGAPORE: Simona Halep and Garbine Muguruza go into the WTA Finals as narrow favourites but the season climax is shaping as a survival of the fittest after a rash of injuries in recent weeks.

With 21-time Grand Slam-winner Serena Williams opting to rest, the tournament has been thrown wide open with injuries especially likely to be the wildcard in Singapore.

Maria Sharapova has entered despite not completing a match since July, while world number two Halep retired in the China Open first round with a recurrence of an Achilles tendon problem.

The formidable Muguruza is the form player after winning in Beijing to reach a career-high world number four, but she is also carrying thigh and ankle problems.

“Every match is going to be close,” Halep said Saturday on the eve of the tournament.

Asked about her own fitness at the end of a gruelling WTA season, Halep said she had recovered to the point where “now I play without pain”.

“I hope it's going to be okay but I have to be careful,” she added.

The top seed has been drawn into what she described as a “tough group” that includes Sharapova. Halep has not beaten the Russian in five meetings, but said Saturday: “I know how to play against her now.”

Agnieszka Radwanska of Poland and Italian veteran Flavia Pennetta are the other players in the Red group, while Muguruza tops the White group featuring 2011 champion Petra Kvitova of Czechoslovakia, Germany's Angelique Kerber and Czech Lucie Safarova.

Sharapova said she felt good going into Singapore but was still concerned about her health. “In this particular situation, I have to worry about myself more than who I'm playing against,” she said.

Muguruza, who will also play doubles with fellow Spaniard Carla Suarez Navarro, admitted she was nervous ahead of her WTA Finals singles debut and didn't consider herself one of the favourites.

“It's going to be a challenge to play singles and doubles,” she said. But she clearly basked in her newfound status as a force to be reckoned with on tour.

“I wanted this to happen, I worked for this, and it's great,” she said.

Pennetta, who shocked Serena at Flushing Meadows in September, will be making her first singles appearance at the WTA Finals after winning the 2010 doubles title with Gisela Dulko of Argentina.

Pennetta announced her retirement this year after the US Open but subsequently said she may compete in the 2016 Rio Olympics.

The WTA Finals features the top eight singles players competing for the Billie Jean King trophy and the top eight doubles teams gunning for the Martina Navratilova trophy.

This year's event boasts a record $7 million in prize money.