Brilliant response

Radwanska next faces 15-times Grand Slam champion Williams, who has yet to drop a set at the tournament.

Radwanska, who had lost to Sharapova in a three-hour and 12 minute marathon on Wednesday, had to pull out all the stops to get past the determined Errani.

Victoria Azarenka of Belarus celebrates defeating Li Na of China during day four of the season ending WTA Championships Tennis at the Sinan Erdem Dome in Istanbul, Turkey, on Oct 26, 2012. [Photo/CFP]

The Italian won the first set 8-6 in the tiebreak after coming out ahead in a 34-ball rally and pumped her fist in delight while Radwanska angrily threw her racket to the ground.

Errani fought off four break points to hold for a 5-4 lead in the second but Radwanska responded brilliantly, winning 12 of the next 13 points to take the set 7-5.

"When I'd played for so long already I was thinking, just pretty much fight until the end," Radwanska said.

Errani troubled the Pole with a mix of heavy topspin and relentless retrieving but was unable to stay as mentally focused as the 2012 Wimbledon finalist, who nearly squandered a 3-0 lead in the third set but finally managed to defeat the Italian.

"She's a big fighter so she didn't give up," Radwanska said. "So I really had to play my best tennis until the end of the match."