Caroline Wozniacki will meet Serena Williams in the semi-finals of the US Open after beating Andrea Petkovic in straight sets.

The match was moved to Court 13 after the surface of Louis Armstrong Stadium was found to be damp from Tuesday and Wednesday's rain, but the change didn't distract the world number one, who took the first set 6-1.

Petkovic proved more resistant in the second and Wozniacki needed to win the tie-break 7-5 to finish the match.

The women's semi-finals will take place on Sunday after a rescheduling of the draw due to the recent weather disruptions.