Lee Westwood headed the 11 European players in the top 19 after the opening day of the 96th US PGA Championship, but Rory McIlroy was ominously poised on a crowded leaderboard at Valhalla.
Westwood equalled his lowest round in any major with an opening 65 to share the lead with American pair Kevin Chappell and Ryan Palmer on six under par.
But Open champion Rory McIlroy, seeking his third win in succession and a fourth major title, recovered from hooking his second shot to the 10th out of bounds and three-putting the 11th to birdie the next four holes in succession and record another on the 18th.
"The second shot on 10 was actually a bit of a shock, it's the worst shot I have hit the last few weeks," McIlroy said after his 66, his 11th consecutive sub-par round. "It came out of the blue and knocked me off track but to come back the way I did was great."
The hype surrounding McIlroy has been intense since he followed his victory at Royal Liverpool with another in the WGC-Bridgestone Invitational on Sunday, with even Jack Nicklaus quoted as saying the 25-year-old could surpass his record of 18 majors.
"People are going to get asked about me. They are going to give their opinion. If that's Jack's opinion, he has a high opinion of me," McIlroy added.
"I've always said I'm on three (majors), I want to get to four. Hopefully I can get to four this week and then keep going from there."
Westwood is still seeking his first major title at the 66th attempt, but also recovered from a double bogey on his 10th hole of the day with five birdies in his last six holes.
The 41-year-old had missed the cut in four successive events before a 63 in the final round in Akron last week and said: "Last week was a big week for me.
"I felt like I had turned the corner and I was starting to swing it a lot better but that's no good unless you start converting it into low rounds.
"The first three rounds were frustrating because I played a lot better than three over. Then obviously I got it going the final day and shot 63.
"I tried to keep it low-key in the practice rounds here, just playing nine holes each day, and carry that momentum through to the first round, which I've managed to do."
Italy's Edoardo Molinari, Sweden's Henrik Stenson and England's Chris Wood were alongside McIlroy on five under, with Finland's Mikko Ilonen four under. Ryder Cup hopefuls Joost Luiten, Ian Poulter and Shane Lowry were all three under along with Danny Willett and Bernd Wiesberger.
Sergio Garcia, Justin Rose and Luke Donald finished one under, although Donald had been just one off the lead before dropping four shots in his last five holes.