( 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 . Fed to face del Potro . Meanwhile at the ATP World Tour 500 event in Switzerland , Roger Federer is safely through to the final of the indoor event in Basel . The Swiss star who is currently No.6 in the world rankings will face Argentina 's Juan Martin del Potro on Sunday after beating Canada 's Vasek Pospisil 6-3 , 6-7 , 7-5 on Saturday . Del Potro also needed three sets ( 6-4 , 4-6 , 6-2 ) to overcome France 's Edouard Roger-Vasselin . At the other ATP World Tour 500 event in Valencia , David Ferrer will face Russia 's Mikhail Youzhny in Sunday 's final . The Spanish star overwhelmed compatriot Nicolas Almagro 6-2 , 6-3 in Saturday 's semifinal while Youzhny beat his fellow countryman Dmitry Tursunov 6-2 , 6-4 .
World No.1 reaches final with three - set win over Jelena Jankovic . Williams will face China 's Li Na for first prize of over $ 1 million . Li 's title win lifts her to World No.3 -- the highest - ever ranking for an Asian woman . Williams advances through to ATP Tour final in Basel , Switzerland .