TORONTO (AFP) - American Serena Williams reached the semi-finals as she rallied from a first set loss to defeat Lucie Safarova 4-6, 6-3, 6-4 at the $2.05 million WTA hard-court tournament here on Friday. The unseeded Williams, who is continuing her comeback from injury and illness, next faces fourth-seeded Victoria Azarenka of Belarus, who breezed past world number 135 Galina Voskoboeva of Kazakhstan with a 6-1, 6-2 win. Australia's Samantha Stosur also advanced to the next round easily beating Roberta Vinci -- the conqueror of world number one Caroline Wozniacki -- 6-4, 6-1. Stosur broke Italian Vinci's serve while leading 5-4 in the first set to take the opening set at the Rexall Centre facility. The Aussie then stepped up her game in the second set and moved through to the semi-finals with authority. "As the match went on, I obviously probably grew in confidence, and then you get that bit of a lead with a buffer of a couple breaks and felt like I could maybe start going for a bit more," Stosur said. "I went for some big ones and it paid off." Stosur next faces Poland's Agnieszka Radwanska, who beat German Andrea Petkovic 6-4, 6-3. "Semi-finals, you've got to believe that anything in possible," Stosur said. Azarenka is the highest seeded player left in the field.