Rory McIlroy walked off a niggling left knee injury to stride imperiously into the lead with a sensational nine under par 63 in the Memorial Tournament at Muirfield Village last night.

After putting his painful break up with Caroline Wozniacki aside to win the BMW PGA with a 66 on Sunday, the world No 6 left host Jack Nicklaus shaking his head in disbelief as he tore the Golden Beat’s slightly rain-soften course to shreds to lead by three shots from Paul Casey, Chris Kirk and Masters champion Bubba Watson.

While claiming to have no idea exactly what he'd done to his knee when hitting his approach to the seventh hole — contradicting his caddie JP Fitzgerald who reportedly told an on-course commentator he'd tweaked it in the gym — the former world No 1 was delighted to take up where he left off at Wentworth.

"I don't think it's anything more than just being confident with my game," McIlroy said, denying any post-Wozniacki factor is at play. "I was expecting this to happen. I've been playing well. I've been posting good numbers, good rounds. And I knew my game was close. And honestly, I don't think it's anything to do with what's happening off the golf course. It's just I've been trending in the right direction, and it's starting to all come together."

Asked when his game started to come around this year, he said: "Basically since before the Masters or before that. I had a couple of chances to win early in the season and didn't take them. So Houston, Masters, Quail Hollow, TPC, all solid top-10s without getting myself into contention.

"Then the first time I got myself into contention last week, I was able to finish it off. So I have a lot of confidence out there and I am just trying to keep that confidence going. 

"The driver has been the big turnaround in my game following the struggles of last year. It set up the two eagles today and it's a course where you need to drive it well. It's great that I have that shot in my bag again and I am confident off the tee."

Bursting with confidence, he made two eagles, seven birdies in a round featuring just 22 putts — 12 on the front nine and 10 in a homeward 31 that even saw him make a somewhat unfortunate double bogey six at the 14th. He also single-putted the last six greens. According to ShotLink, he holed all 16 putts he faced inside 15 feet, an incredible performance for a player who missed more than a dozen putts from that distance during the Masters.

McIlroy was visibly limping when he tweaked his left knee hitting a three-wood at the par-five seventh but said he was unsure how the injury occurred.

"Honestly, I don't know. I felt it on my second shot on seven today. I think my foot might have got stuck in the ground a little bit and I 'twerked' my knee a little bit. I'll go and have treatment on it now and have the physio look at it. 

"I was painful right after I hit shots and then I walked it off for the next one and it was okay and then it was painful again. A little bit of ice and treatment and hopefully it will be fine tomorrow."

Asked if it was a new injury or a tweak of an existing injury, he said: "This is a new injury. I haven't felt this before. I haven't struggled with my knees before. If anything I have struggled with my back. I tweaked my back at Wentworth on Friday. So I am struggling a little bit with that. But the knee, I don't know where it has come from. So I will just have to see the physios and let them work on it."

Explaining how he managed to shoot 63 when in pain, he said: "I guess I put it out of my mind when I was over the golf ball and pretended it wasn't there and just tried to play the best shot that I could. 

"I knew that it was going to be a little sore after shots, especially if I went after an driver or trying to hit an iron shot into a par-five. I just walked it off. But I obviously played really well from tee to green, holed a lot of putts as well. I made two really good up and downs at the last two holes. It could have been ever better but for the double bogey on the 14th but obviously a really good opening round." 

The Ulsterman was already two under after holing a 10 footer at the second and a nine footer at the fourth when he hurt his knee at the seventh. But even though he fanned his three-wood approach wide of the crowd, he holed a six footer for birdie and then made six threes on the spin.

After holing a 14 footer for another birdie at the ninth to turn in 32, he politely turned down an offer of treatment before continuing his rampage.

He birdied the 10th from four feet and eagled the 11th, reducing the 570-yard par-five to a drive, a mid iron and a 27 foot putt before making another birdie at the 13th to get to eight under par.

Two clear at that stage, he was forced to accept a double bogey six at the 14th, where he pulled a wedge and plugged his approach in a bunker with a wedge and left himself with no shot.

But he showed that he is almost bulletproof right now by bouncing back with another eagle three at the 15th, hitting a 205 yard approach to just 15 feet to go two in front again.

Within moments his lead was three as he rifled a 191-yard tee shot to 13 feet at the short 15th and rolled in the putt to get to nine under.

Phil Mickelson was five under with three to play but finished bogey, double, double to card a 72 that left him nine behind McIlroy. 

And the Irish star was clearly chuffed with his score and the way his game has progressed this year.