Unwell Jankovic to face Serena

Jelena Jankovic is hoping her efforts to derail Serena Williams' march towards a fourth WTA Championships title will not be snuffed out by a cold.

The Serbian knew before her final round-robin match against Sara Errani that she was already through to the semi-finals but she was clearly under the weather as she went down 6-4 6-4.

Jankovic has a decent record against defending champion Williams having won four of their previous 11 matches, but the American has looked unstoppable so far.

Jankovic said: "I felt really flat-footed out there, low on energy and I did not have that intensity I needed to play these kind of matches. Hopefully I can have a good night of sleep and feel better tomorrow.

"You have to have the belief that you can win against Serena. That's the number one thing, because she has that intimidation factor.

"She's a great champion. She has a lot of knowledge. In order to beat her you have to play your best tennis, have belief in yourself and really fight for every point."

The other match will pit first-time semi-finalist Li Na against 2011 champion Petra Kvitova.

Li won her third round-robin match on Friday against an ailing Victoria Azarenka, who suffered a back spasm in the first set.

The second seed was barely able to move at times but she opted to finish the match, which was predictably one-sided as Li took it 6-2 6-1.

Azarenka hinted afterwards that her reputation for withdrawals - she has pulled out before or during matches 25 times during the past eight seasons - made her reluctant to do so again for fear of criticism.

She said: "There is a no-win situation, retiring and it being discussed.

"I don't have a tournament next week. The physio told me there is no real structure damage but you can't fix it that quickly."

Li has had her most consistent season, reaching at least the quarter-finals of every tournament she has played apart from those on clay, and she will finish the year at number three in the rankings if she beats Kvitova.

The 31-year-old said: "I think this is the best year of my career but the tournament isn't finished here so I have to still look forward."

Kvitova fought back from a set down to defeat Angelique Kerber 6-7 (3/7) 6-2 6-3 and secure second spot behind Williams in the red group.

She and Li have played each other six times, splitting them three each, with Kvitova winning their most recent clash in Beijing earlier this month.

The Czech said: "She's played really well here. I watched some matches when I was waiting for my match and she's on fire.

"I played her in Beijing, it was a pretty close match, every game was close. I was just lucky and I think it's going to be a big fight, a nice match for the people and for us, too."