McIlroy produced an imperious display to get to six-under in Tulsa, before overcoming a late blip to seal a closing birdie that set the clubhouse lead with a five-under-par 65.

"I came in here knowing that my game was in good shape," McIlroy told Sky Sports. "So it's just a matter of going out there and executing the shots that you know that you can.

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"Today I did that very well and I just need to try to replicate that tomorrow, Saturday, Sunday and not get ahead of myself, but it was a great start."

The performance ended a streak of disappointing opening-round displays at majors for McIlroy, who had posted a combined 35-over par in the first round of majors since the most recent of his four major victories, at the 2014 PGA Championship.

It was a continuation of the form McIlroy showed in his most recent round at a major, when a final-day charge at The Masters in April saw him earn a second-place finish behind Scottie Scheffler.

"I've been playing well coming in here," McIlroy said. "I've been carrying some good form. Obviously I took a lot from that last round at Augusta, played well at the Wells Fargo, and played good in the practice rounds earlier this week.

"I think when your game is feeling like that, it's just a matter of going out there and really sticking to your game-plan, executing as well as you possibly can, and just sort of staying in your own little world. I did that really well today. It was nice to get off to that good start and keep it going."

'So nice to end with birdie'

Having surged into a three-shot lead with a birdie to get to six-under at the fifth (his 14th after teeing off at the 10th), McIlroy dropped shots at the challenging par-three sixth and eighth holes to find himself tied at the top of the leader-board.

However, he regained his composure to set up a birdie chance at the ninth, and holed a 19-foot putt to re-establish his position at the top of the standings.

"It was so nice, because to get to six-under for the day and then give a couple back on the par threes, without feeling like I did that much wrong," McIlroy said.

"It's that sort of golf course, you don't have to do much wrong to make a bogey around here.

"So, to get one of those shots back and finish with a birdie, it's always nice."

McIlroy explained that his comfort levels were aided by playing alongside Tiger Woods (+4) and Jordan Spieth (+2), who both struggled on Thursday.

"Once the draw came out on Tuesday, I was excited to play in a group like that," McIlroy said. "I always enjoy playing with Tiger and Jordan.

"It's a pretty comfortable group for all of us, we've all played together a lot, we all know each other pretty well."

Faxon: Rory needed good start to feel better about himself

McIlroy's putting coach Brad Faxon, who is part of the Sky Sports commentary team at Southern Hills this week, believed it was of vital importance for the two-time champion to make a strong start.

"I know that was on Rory's mind, trying to get off to a good start," Faxon said. "I don't think he needed to lead the tournament to feel better about himself, just get himself where he was under par, and what a great way to start.

"He made that five-under look very easy - putted beautifully, drove the ball far, and I like his short game right now.

"He did it with some beautiful drives, whether they had to curve to the left or to the right, short holes and long holes, his iron play looked pretty good too. That approach play of his, that might have been a little bit in need of some improvement, certainly looks good too.

"He holed some nice putts for par and even though he dropped two shots he made that one at the last and that's going to make lunch taste way, way better."