South Africa's Jaco van Zyl and England's Callum Shinkwin were a shot further back, with defending champion Soren Kjeldsen, two-time major winner Martin Kaymer, 2010 champion Ross Fisher and Brett Rumford on three under.

Fisher and Kaymer were the only players in the first half of the field to break 70 as heavy downpours lashed the 2006 Ryder Cup venue before giving way to bright and breezy conditions in the afternoon.

"It did not look pretty when I opened the curtains and I think today was a tale of two halves really," said Willett, who missed the cut by a shot in the Players Championship last week in his first event since Augusta.

"Three under this morning was a fantastic score and to be able to do four better this afternoon I'm very happy.

"Last week didn't quite go as we planned but we were still a little bit tired, a little bit rusty with all the things we had to do. This week has been trying to get back to what I do and try to get grinding and it's obviously paid off thus far.

"I got off to a pretty scrappy start but got a bit hot with the putter on the back nine and holed a lot of footage. You can't really go out there and expect to shoot any kind of number and 65 was far beyond my expectations for today."

McIlroy has recorded just two top-10 finishes in nine appearances in the Irish Open and was frustrated at what he felt was "negative spin" being put on his failure to win so far in 2016.

However, the 27-year-old admitted the prospect of getting that win in front of his home crowd gives him "goosebumps", adding: "To win here, no matter what the circumstances are, whether I've won the week before or whether I haven't won in six months, would be something be very special and I've got off to a great start towards doing that this week. Just have to keep playing the way I did today.

"We definitely got the better of the conditions and I think there was more out there. I thought it was a very comfortable 67. I got to five under after 13 with a couple of fives to come in and had a bit of a blip on 14 with a three-putt. But apart from that, I felt like I played pretty well."

Kaymer, who has slipped to 64th in the world rankings after two years without a win, was delighted with his score of 69 and revealed he had spotted a flaw in his swing on television during last week's Players Championship, an event he led from start to finish in 2014.

"It was very difficult to hit the fairways because you had so much water on the clubface," the 31-year-old said. "Overall, shooting three under par is a great round.

"I played really good golf today. I gave myself a lot of chances and missed three or four putts within six, seven feet and still shot three under par, which obviously says a lot about the ball-striking.

"I was very happy the way I hit my drives. They were long and straight, and that's always a positive.

"In Sawgrass last week I found something on Sunday because my clubface was too open on my backswing. It's a very little change but sometimes you're not aware of that, and I saw it on TV. I just fixed it on Sunday and it felt good."