The Northern Irishman, who has recorded just one top-10 and six missed cuts in 18 starts worldwide during his 2015 campaign, ground out a four-under-par 66 to end the opening round at a firm and fiery Firestone Country Club one shot off the lead.

"I drove it well today, especially on the front nine," McDowell told reporters after mixing five birdies, four of them in his first seven holes, with one bogey. "Struggled a little bit on the back nine.

"The golf course is a pretty tricky set-up this year. The rough's pretty gnarly and the greens are pretty firm, so a real premium on accuracy off the tee and coming into these greens. I really hung in there well with some short game coming in."

Former U.S. Open champion McDowell, whose world ranking has slipped to 60th after he began the year in the top 20, said he had asked himself a few soul-searching questions in a bid to resurrect his game.

"It's been a rough year, no doubt about it. Definitely been some time for reflection and some questions being asked of myself," he said. "It's how you answer the questions and how you come out the other side really.

"I've just been working hard on the game and trying to find a direction forward. I feel like I'm starting to do that now, and it's just a case of shooting some numbers and getting the confidence back a little bit and getting the juices flowing.

"I feel like I'll know what to do when I get back there, but it's all about the process of getting there now," said the Northern Irishman, who won the 2010 U.S. Open at Pebble Beach. "Days like today will certainly help in that direction."

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