Serena Williams of the US hits a return to Agnieszka Radwanska of Poland during their WTA tennis championships match at Sinan Erdem Dome in Istanbul October 23, 2013. [Photo/Agencies]

ISTANBUL - World number one Serena Williams beat Agnieszka Radwanska for the eighth time in eight matches between them to move ominoulsy towards the semi-finals of the WTA Championships on Wednesday.

The American was again too strong for the Pole in a 6-2 6-4 victory to stay in charge of Red Group with one more round-robin match to come against Petra Kvitova.

Radwanska, who has managed only one set in her meetings with the 32-year-old Williams, is now unlikely to survive the group after two defeats so far at the season-ender.

Earlier in White Group Serbia's Jelena Jankovic beat second seed Victoria Azarenka 6-4 6-3, aided by 40 unforced errors from her erratic opponent.

"It feels incredible to beat Azarenka here in Istanbul," former world No.1 one Jankovic, making her first appearance at the event for three years, said.

"I was so excited to qualify and give myself a chance to play against the best players in the world once again, and to win today is amazing.

"Every match in the round robin format counts, and every match is difficult too. I'm really happy. Beating Vika will give me a lot of confidence going into the next matches."

Azarenka had won her opening match against Sara Errani on Tuesday and will have a day off on Thursday, while Jankovic will play her second round robin match against China's Li Na.

Li beat am injury-hit Sara Errani 6-3 7-6(5) to raise hopes of surviving the round-robin stage after failing to do so in 2011 and 2012.

"It was important to win the first match because it is always tough. Sara was fighting a lot today, she is always a tough fighter," said Li.

Errani, who had her calf heavily strapped, has lost both matches so far. (Editing by Martyn Herman)