Marc Leishman of Australia set the early pace at the end of the first round of the WGC-Bridgestone Invitational in Akron on Thursday after shooting a blistering round of 64.
Rory McIlroy had to settle for an opening 69 on his return to action after claiming his third major title at the Open last month.
McIlroy three-putted the third hole - his 12th - and ran up a double-bogey six on the eighth after finding a greenside bunker with his approach and taking four more shots to get down.
However, he finished in style with a brilliant approach to the ninth to set up a birdie from just two feet to finish one under par.
"I didn't want any sort of let down and not just these two weeks, but basically the rest of the season because I've got myself now in a good position in the FedEx Cup [12th], a good position in the world rankings, we've got a major left this year and there's a lot of golf still left to play," added McIlroy, who could reclaim the world number one spot from Adam Scott with his first World Golf Championship victory on Sunday.
"It's great to win a major, great to win the Open Championship but I just want to keep going forward and I'll have plenty of time in December and January to reflect on it and enjoy what I've achieved this year.
"For now I want to keep going and try to have some more success on the course."
Graeme McDowell opened with a one-over-par 71 to sit seven shots off the lead.
The former US Open winner recovered from a run of three bogeys in four holes to stay well-placed going into today's second round.
Justin Rose gave himself a belated birthday present with a brilliant opening round. Rose, who turned 34 on Wednesday, carded a flawless five-under-par 65 to lie just one shot behind Leishman.
The Englishman, who won back-to-back events for the first time in his career before the Open Championship, birdied the second and ninth to be out in 33 before picking up further shots on the 15th, 16th and 18th.
"I'm really happy with that number because it wasn't the most in-control round of golf," Rose said.
"The scorecard will tell you something different but I got into some trouble a couple of times and round these greens you can get some gnarly lies where it isn't a simple up and down, even if it looks like it.
"I kept the scorecard clean with a really good mix of some good iron play and good scrambling when I missed and that's what you have to do round here.
"I'm beginning to play really, really well and build up some confidence. I think that win at Congressional [in the Quicken Loans National] came at exactly the right time.
"I had a slightly slow start at the beginning of the year and couldn't find the gear I was looking for and then boom, it happened immediately. That's why you have to stay patient in this game.
"It's a good time to play well, there are lot of big tournaments coming up so I'm happy I've found the right gear at the right time."
Rose joined American Ryan Moore and former Masters champion Charl Schwartzel in second place, Moore also shooting an error-free 65 to improve his chances of securing a Ryder Cup berth.
The 31-year-old began the week 14th in the points standings with the top nine after next week's US PGA Championship sealing a place on Tom Watson's team for Gleneagles, but has already effectively moved up a place with fifth-placed Dustin Johnson taking a "leave of absence" to deal with personal problems.
"For me, I've got to earn a spot and I know it," Moore said. "That's how I'm looking at it. I got two more weeks to do that. I'm making it simple on myself and in my mind that I have to earn a spot. I don't want to rely on anything like [being picked], and I would be very surprised if I got a pick."
