Li Na of China hits a return to Jelena Jankovic of Serbia during their WTA tennis championships match at Sinan Erdem Dome in Istanbul, Oct 24, 2013. [Photo/Agencies]

ISTANBUL - China's Li Na outlasted Jelena Jankovic 6-3, 2-6, 6-3 for her second victory in two matches while top-ranked Serena Williams advanced to the semifinals of the WTA Championships by sweeping past Petra Kvitova 6-2, 6-3 on Thursday.

Williams, the defending champion, won all three matches in her group without dropping a set. She's bidding for her fourth title in the elite, eight-women tournament.

The key to Li Na's victory in an error-filled match full of long rallies was her break of serve in the third set for a 4-3 lead. She never looked behind again, winning the last three games. Jankovic hit a backhand into the net on Li's second match point.

The fourth-seeded Li has a 2-0 record in the White Group and looks likely to advance to the semifinals for the first time. Seventh-seeded Jankovic is 1-1. Jankovic is back in the tournament for the first time in three years. The two of them and Victoria Azarenka are vying to make the semis.

"It was tight, yeah, but at least it's better than the last two years," said Li, who won only once in 2011 and 2012. She was pleased to add she had one foot in the semis but her other foot was "still out of the door."

Li holds a 6-4 career edge over her Serbian friend, but she had to work hard for her win. Both players were inconsistent and error-prone. Li had 45 unforced errors and 21 winners, Jankovic 36 unforced errors and 17 winners.

"We had a great fight," Jankovic said. "In the third set it was just a couple of points. I got broken in that (seventh) game. She lobbed me, and the ball went on the line."

Li followed the lob with a crisp backhand winner to take the game.

"They were the crucial points at that stage and made a difference," Jankovic said.

Jelena Jankovic of Serbia falls during her WTA tennis championships match against Li Na of China at Sinan Erdem Dome in Istanbul, Oct 24, 2013. [Photo/Agencies]