(CNN) -- Defending champion Serena Williams overcame a gutsy display from Jelena Jankovic on Saturday to reach the final of the WTA Championships in Istanbul. 

The World No.1 was forced to work hard against her Serbian opponent in the Sinan Erdem Dome, but eventually came through to win in three sets 6-4, 2-6, 6-4 to set up a showdown with China's Li Na in Sunday's final. 

Williams started slowly and was soon 3-1 down but fought back to take the opening set. But any thoughts of a quick finish were dashed as errors crept into the American's game. Jankovic took full advantage winning the set with ease. 

A more composed Serena returned in the deciding set which she won 6-4 but not before Jankovic had gamely saved three match points. 

"I'm just really trying out here. I think my body's a little tired after so many matches -- I think yesterday I kind of hit a wall. But I'm just fighting out here," Williams said. 

"Honestly I'm just happy to still be in the tournament, and I'm honoured to be in the final again." 

Li's match against Petra Kvitova was a less exhausting affair with the 31-year-old seeing off the Czech left-hander 6-4, 6-2. 

Li had already broken new ground by becoming the first Chinese woman to reach the semifinals of the WTA Championships, but by beating Kvitova, she has risen to World No.3 and becomes the highest ranked Asian women in history. 

With both Li and Williams undefeated in the tournament so far, the winner of Sunday's match will walk away with Â£2,145,000 while the runner-up is guaranteed $1,090,000. 
And the runner up?
the runner-up is guaranteed $1,090,000