(CNN) -- The world's third-ranked golfer, Paul Casey, was forced to withdraw from the U.S. PGA Championship ahead of Thursday's opening round due to a rib injury.
Paul Casey decided to pull out of the PGA Championship with a view to getting fit for the end of the season.
The Englishman has been replaced in the field by American Tim Petrovic for the season's final major tournament at Hazeltine in Minnesota.
The 32-year-old tied for 47th at the British Open at Turnberry after straining a muscle in the buildup, and then pulled out of last week's World Golf Championships Bridgestone Invitational in Ohio after just six holes of his opening round.
He played nine practice holes at Hazeltine on Wednesday, but did not feel fit enough to fulfil his 7.45 a.m. tee time alongside Japan's Hiroyuki Fujita and American Nick Watney.
"I'm very disappointed but I have to look at the bigger picture, which is the fact that I will play a lot more majors over my career and I have to be ready for what is coming up in the rest of the season with the FedEx Cup and The Race to Dubai," Casey told reporters.
"But it is very frustrating, because standing there hitting easy golf swings it is not that bad. It is just starting to pull when you are getting to six irons and longer.
"If it is doing that on a flat lie on the range, then I can't risk it on the golf course. It seems like there has been a lot of guys who have had similar issues in the past, and the advice I have been given from the experts and a lot of the players is don't push it too quickly.
"I feel good that I tried everything to try to get it ready, and I also gave Tim Petrovic an hour. He was out there warming up alongside me, so I went to speak to him myself and told him to go and play well."
Petrovic last played in the event in 2005, the only year he has won on the PGA Tour -- claiming victory at the Zurich Classic of New Orleans.
The 42-year-old also missed the halfway cut in his two other starts at the PGA in 2003-04, and has only made the weekend rounds four times in majors with a best finish of equal 15th at the 2003 U.S. Open.