ISTANBUL - American Serena Williams reached the semifinals at the WTA Tour Championships in Istanbul on Thursday with what is fast becoming a routine defeat of world number one Victoria Azarenka.

Serena Willams of the USA returns the ball to Victoria Azarenka of Belarus during their match at the round-robin stage of the WTA Championships tennis tournament in Istanbul, Turkey, 25 October 2012. [Photo/CFP]

The Wimbledon and US Open champion recorded a 6-4 6-4 win at the season-ending event, her fifth triumph over the Belarusian this year. Williams also owns an 11-1 record in their career head-to-heads.

The win was Williams' third at the tournament and while she is guaranteed to top the round robin red group, Azarenka must now beat China's Li Na on Friday to reach the last four.

The eighth-ranked Li ended German Angelique Kerber's participation in the event with a 6-4 6-3 triumph.

Italy's Sara Errani is also still in semi-final contention after a three-set win over Australian Samantha Stosur.

Errani faces a must-win match against Poland's Agnieszka Radwanska if she is to join world number two Maria Sharapova in advancing from the white group.

Williams has won 12 of the 13 sets she has played with Azarenka in 2012. Her earlier wins over the 23-year-old came at Madrid, Wimbledon, the Olympics and the US Open.

Azarenka's power troubles nearly every other player on the WTA Tour but Williams has a substantially bigger serve than most and an equally good return.

Williams was again let down by her serving, but she still managed 35 winners to just 15 for Azarenka, broke her five times and won the match when Azarenka doubled faulted for the ninth time.

"I had too many opportunities that I didn't use," Azarenka said.

"She played really well. I didn't execute well enough. For me, the key word today was discipline, which was missing."

The 31-year-old Williams was at a lost to explain her dominance over Azarenka.

"Maybe I'm doing the right things at the right time." Williams said. "I don't know what the edge is."

Azarenka admitted she struggled mentally against Williams but fancied one more match against her before the year was done.

"She's playing great tennis," Azarenka said.

"She's letting her tennis speak for itself. My biggest opponent is her right now. I'll do everything I can. I might still have a chance to play against her in this tournament (in the final), so we'll see."