No. 1 Serena Williams sealed her place in the quarterfinals of the U.S. Open with a straight-sets victory over Sloane Stephens on Sunday.

Williams, the grand slam’s defending champion, was given a fright by her fellow American at Flushing Meadows, but her experience and quality shone through in the second set. Williams emerged a 6-4, 6-1 victor in 89 minutes.

Stephens defeated Williams in the quarterfinals of the Australian Open in January, and they have become rivals as the two best women in American tennis; Serena the powerful veteran, Sloane the rising challenger.

Williams, 31, looked to be on her way to the first set when she broke in the sixth game with a forehand winner. However, Stephens, 20, the world No. 16, refused to lie down and broke back immediately before holding serve to level matters at 4-4.

Still, despite her best efforts, Stephens was overpowered by Williams, who clinched the opening set in the 10th game as Stephens sprayed a forehand wide.