Na moves to the brink

Istanbul - Li Na has clinched her second win at the season-ending WTA Championships on Thursday, a 6-3, 2-6, 6-3 victory over Jelena Jankovic putting her on the verge of the semi-finals.

Li is already the first singles player from China to have won a Grand Slam title and the first to have reached the world's top five.

Now she is within touching distance of becoming the first to qualify for the semi-finals in the tour's flagship tournament.

It was an excitingly unpredictable contest, with Li having three points for a 4-2 second set lead, before losing five games in a row and a great deal of momentum.

She then also let slip a break of serve at 3-1 in the final set to be pegged back to 3-3.

However, she was often the more forceful player off the ground, and her success now means that if other results go right Li may qualify irrespective of the outcome of her last group match on Friday.

That will be a repeat of the Australian Open final against Victoria Azarenka.

It was hard to explain the swings of an absorbing contest between two of the tour's strongest personalities.

There may have been a hint in Li's comment the previous evening that "I have to focus on the match when my mind is thinking of the vacation."

However another factor was the fighting spirit of former world number one Jankovic, who never knew when she was beaten, and which may also have been a part explanation.

Her extraordinary efforts in winning the point to get her back from 40-love down to deuce in the sixth game of the second set may have had much to do with the emotional swings as well.

Jankovic made four tough retrieves from the back, and then somehow got to the net, finishing the rally as the aggressor and with a huge smile.

After that Li slipped from being close to taking control to the parity of a dogfight in the final set.

Li scored heavily against Jankovic's second delivery, and from 3-3 in the decider that helped her to achieve two breaks of serve and carried her to a fiercely-fought two-hour 11-minute triumph.

With Jankovic favourite to win her final group match against the winless Sara Errani, it is possible that three players could finish with two wins, which would mean a count-back of the percentage of sets won to decide who are the two qualifiers.

Angelique Kerber, the last player to qualify for the WTA Championships, kept alive her against-the-odds chances of surviving into the semi-finals after a surprise win.

The Poland-based German overwhelmed Agnieszka Radwanska, the third-seeded Pole who reached last year' Wimbledon final, by 6-2, 6-2 in less than an hour.

Radwanska is one of only two players with more than 50 wins on the tour this year -- Serena Williams is the other -- and this seemed to be one match too far.

Her movement was laboured and her will to influence the outcome of a difficult match ebbed visibly in the second set. After three losses in Istanbul, her season is over.

Kerber still has an outside chance of reaching the last four from her section although that would require a solid win over Petra Kvitova, the 2011 champion, on Friday.