Caroline Wozniacki, seeking a first Grand Slam title to back up her number one ranking, booked a US Open semi-final showdown on Thursday with 13-time major winner Serena Williams. Denmark's Wozniacki, whose runner-up finish at Flushing Meadows in 2009 remains her best Grand Slam finish, survived a hiccup while serving for the match at 5-3 to beat determined German Andrea Petkovic 6-1, 7-6 (7/5). The match was moved from the water-logged Louis Armstrong Stadium to the much smaller court 13. The contest had already been postponed from Wednesday, when heavy rain washed out play for the second straight day. Petkovic, who was in her third Grand Slam quarter-final of the year after reaching the last eight at the Australian Open and Roland Garros, was denied a first trip to a major semi-final.