Ailing Serena Williams struggles vs Jelena Jankovic; will play Li Na in WTA final in Istanbul

Serena Williams of the USA returns a shot to Jelena Jankovic of Serbia during their semifinal tennis match at the WTA Championship in Istanbul, Turkey, Saturday, Oct. 26, 2013. The world's top female tennis players compete in the championships which runs from Oct. 22 until Oct. 27.(AP Photo) (The Associated Press)

ISTANBUL – Serena Williams, in obvious discomfort and walking slowly during changeovers, struggled past Jelena Jankovic 6-4, 2-6, 6-4 on Saturday and will play Li Na in the final of the WTA Championships.

Li advanced to her first WTA Championships final by sweeping 2011 winner Petra Kvitova 6-4, 6-2, setting up a showdown Sunday between the two oldest players in the tournament.

It's only the second time in the history of the event that two players over 30 — Williams at 32 and Li at 31 — have made it to the last four. The season-ending tournament brings together the top eight players in the world.

It was not clear what was bothering Williams. She did not move well on the court and frequently held her head wrapped in a towel during changeovers. Williams tried to keep the points short and often winced.

Jankovic was unable to take advantage of whatever was bothering Williams, who is one win from her fourth WTA Championship and 11th title of the year.

Williams wasted a match point and failed to serve out the match at 5-2, and Jankovic won two straight games. Williams squandered two more match points before finally closing it out with a smash.

By reaching the final, Li will improve two places to a career-high No. 3 ranking. It's the highest ranking for an Asian woman.

Like Williams, the Chinese player is unbeaten in four matches, after failing to advance from the group stage in the previous two years.

Kvitova, the youngest player in the tournament at 23, trailed 3-0 before tying the match at 4-4.

But a backhand winner from Li broke Kvitova again and she closed out the set with a service winner.

Kvitova had no reply to Li's forehand and was broken again in the second set to fall behind 3-2. After saving a break point, Li hit an ace to go up 4-2.

Kvitova dropped serve again and netted a backhand on Li's second match point for her 29th unforced error.

Li improved her career record to 4-3 against Kvitova, who won Wimbledon in 2011 — the same year Li won the French Open.