Serena Williams has beaten world No. 1 Victoria Azarenka in straight sets to top the red group at the season-ending WTA Championships in Istanbul.

Williams has won all three matches of the group stage of the $4.9 million tournament.

Her semifinal opponent will be the winner of Friday’s clash between Poland’s Agnieszka Radwanska and Sara Errani of Italy.

Azarenka must now defeat China's Li Na on Friday to avoid an early exit.

Williams, who has won the WTA Championships twice, stretched her head-to-head record against Azarenka to 11-1 after completing a 6-4, 6-4 win.

"What I felt was really positive about my match is I know I can do better," Williams said on the WTA website.

"I've been practicing really hard, and I actually expected to play a bit better and hopefully I can as the tournament goes on."

Azarenka kept up with Williams for most of the first set but crumbled at the end, giving Williams the crucial break with her fifth double fault of the set.

Azarenka broke Williams twice in a second-set surge before the American won four games in a row to retake control.

The Belarusian top seed made 22 unforced errors and nine double faults and blamed a lack of concentration at key moments.

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

"I feel like I didn't keep my discipline. When I had the right play, I was in control most of the times, but I just couldn't do it consistently today."

Elsewhere, Sara Errani had to dig deep to outplay Australia’s Sam Stosur, who replaced the ill Petra Kvitova at the tournament.

The Italian, in her debut at the WTA Championships, suffered a setback in the second set but stormed back in the decider for a 6-3, 2-6, 6-0 win.

Stosur’s aggressive style was undermined by mistakes as she racked up 49 unforced errors against Errani’s 15 and double-faulted both times she was set point down.