Serena Williams clinched her third win at the WTA Championships on Thursday with a 6-4, 6-4 victory over Victoria Azarenka that prevented the Belarusian from guaranteeing the end-of-year number one spot. World number three Williams, who had defeated Azarenka in the US Open final last month, had already made sure of her place in the semi-finals after beating Li Na and Angelique Kerber. On Thursday, she hit 35 winners and 11 aces. Azarenka must beat Li Na on Friday to make sure of her place in the semi-finals of the season-ending event.