Rafael Nadal and Novak Djokovic stayed on course for a French Open semi-final showdown on Friday as Serena and Venus Williams both crashed out of Roland Garros at the third round stage.While the American sisters, with 14 Grand Slam titles between them, were packing their bags, women's top seeds Maria Sharapova and Ana Ivanovic remained on track.Nadal, bidding to emulate Bjorn Borg as the only man to win four successive Roland Garros titles, needed treatment on his blistered right foot before brushing aside Finland's Jarkko Nieminen 6-1, 6-3, 6-1.He will now face compatriot, and fellow left-hander Fernando Verdasco, for a place in the last eight.