McIlroy made two eagles and three birdies on the back nine at Muirfield Village -- along with a double-bogey -- on his way to a 9-under 63 and a three-shot lead Thursday after the opening round of the Memorial.
A week ago, McIlroy began his week at Wentworth by announcing he and tennis star Caroline Wozniacki had broken off their engagement. Then, he went out and won the BMW Championship for his strongest win in more than two years.
At the Memorial, McIlroy caught his spikes in the turf on his second shot at the seventh hole and felt pain the rest of the round, especially when he had to put a little extra pop into tee shots or long irons. He was limping when he walked off the stage after talking about how he made five birdies and two eagles after hurting his knee.
It was the lowest opening round in 39 years at the Memorial.
"I don't think it's anything more than just being confident with my game," McIlroy said. "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."
Masters champion Bubba Watson, Chris Kirk and Paul Casey each had a 66 while playing in calmer and slightly softer morning conditions. Keegan Bradley was at 67, notable not because it was his best score at Muirfield Village but because it was his first time in his four years on the PGA Tour that he did not use a belly putter.
Adam Scott, who won at Colonial last week in his debut at No. 1 in the world, lost momentum with a shot into the water for double-bogey on No. 9, and he missed some birdie and eagle chances on the back nine that would have allowed him to stay close with McIlroy. He settled for a 69.
Phil Mickelson also looked sharp -- at least for 15 holes. He was at 5 under after a birdie on the 15th hole -- at the time, just one shot out of the lead. But he hit into the water on the par-3 16th and did well to escape with bogey. He came up short on the 17th and muffed two flop shots on his way to double-bogey. And he finished with another double-bogey by three-putting the 18th. That gave him a 72.
"Just threw away a good round," Mickelson said.
The course began to get firm under a hot sun late in the afternoon, which didn't appear to bother McIlroy. Jordan Spieth picked up birdies on the par 5s, had a good par save on the 16th and squeezed out a 69 that left him pleased.
"Anything under par is a good score," Spieth said. "Obviously, Rory didn't think that."
McIlroy was more than nine shots better than the field average. His only scare came on the seventh hole when his spikes caught and applied a little more torque on his knee. McIlroy said he had never experienced that before, although he thought some ice and work in the fitness trailer should take care of it.
"The inside of it is sore, a little swollen," McIlroy said. "Some treatment, and I should be fine for the morning."
The golf after that was simply spectacular.
From the deep rough right of the 10th fairway, he hammered a shot to the middle of the green, and it caught a slope and settled 4 feet away. He hit a 4-iron to 25 feet on the par-5 11th and holed a 25-foot birdie putt in the center of the cup. He holed a tough 15-foot birdie on the 13th and then hit a towering 5-iron to 15 feet on the par-5 15 for another eagle. McIlroy picked up his final birdie with a 12-foot birdie after a great tee shot to the deceptively tough front pin on the par-3.
He finished his round with an 8-foot putt to save par from the bunker.
His only blunder came on the 14th when he tugged a wedge too hard and too far left into a plugged lie in the bunker. He played well away from the flag to avoid going down the slope and into the water but left it in the sand. His next shot had to be good to get within 6 feet of the flag, and it wasn't. The ball ran off the green and down a slope. He had to get up-and-down for a double-bogey.
McIlroy answered by smashing another tee shot into the fairway and hitting that high 5-iron onto the green for eagle, erasing the double-bogey.
It's just one round, but considering it was McIlroy, it was an attention-getter. McIlroy is one of those players who can follow up a win with more great play. When he won the PGA Championship by eight shots in 2012, that was the start of three victories in four starts against the strongest fields of the year.