The Williams sisters are frequently recognized for having a transformative impact on tennis through the power in their style of play, their longevity, their marketability, and their success at the Grand Slam tournaments and Olympics. They were both No. 1 in the world and combined for 15 Grand Slam singles titles while Clijsters was on tour, and Serena in particular has been ranked as the greatest women's tennis player of the Open Era. Clijsters had a record of just 2–7 against Serena, who she called her toughest opponent. She fared better against Venus, compiling a winning record of 7–6.