Ruthless Serena Williams underlined her dominance at the top of women's tennis on Sunday, completing her year in style with a 6-2 7-6 defeat of big sister Venus in the final of the WTA Championships. The two Americans, facing each other across the net for the 22nd time in their glittering careers, provided nearly an hour and a half of power tennis under the Khalifa Stadium floodlights but there was only going to be one winner.Serena, who came to Doha ranked second in the world behind Dinara Safina but will return home as the rightful number one after winning both the Australian Open and Wimbledon titles, broke serve twice in a one-sided first set.Defending champion Venus did make more of a match of it in the second set but never got close to breaking the Serena serve that leaked just seven points in the entire match.Serena pressed the accelerator in the tiebreak to build a 5-1 lead and although Venus clawed back three points, the 28-year-old completed victory with her fifth ace followed by a fizzing forehand into the extremity of the court.