World number three Paul Casey of England withdrew from the 91st PGA Championship on Thursday morning because of a nagging rib muscle strain.

Casey, suffering from a strained intercostal muscle, was unable to play a practice round at Hazeltine but said he felt well after swings at the practice range. He received treatment for the injury the past two days.

After delaying a decision from Tuesday to Wednesday and then to the final minutes before his scheduled tee time, Casey determined that the injury was too much for him to take part in the year's final major tournament.

The 31-year-old Englishman withdrew after six holes from last week's World Golf Championships Bridgestone Invitational after straining the rib muscle that he first hurt last month while preparing for the British Open.

Alternate Tim Petrovic joined fellow American Nick Watney and Japan's Hiroyuki Fujita in the morning start off the 10th tee.

Casey, who trails only Tiger Woods and Phil Mickelson in the world rankings, won titles this year at the Abu Dhabi Championship and the BMW PGA Championship on the European PGA Tour as well as the US PGA Houston Open.