Rory McIlroy has explained his reasons for reverting back to a conventional putting stroke at the Memorial Tournament, just two weeks before the U.S. Open.

Four-time major champion McIlroy adopted a new technique, which saw him putt with his left hand below his right, in March and claimed victory on his most recent start at the Irish Open two weeks ago.

However, in round one of the Memorial on Thursday, the Northern Irishman utilised his previous grip throughout a topsy-turvy round of 71.

Asked about the change, McIlroy said: "I had 127 putts the week that I won the Irish Open. So it's not as if I putted particularly well that week. I won that golf tournament with my ball striking alone.

"I knew I needed to make a change and I felt like my pace wasn't really good with left hand low, especially going into this week. Obviously, the greens are fast.

"But then going into Oakmont [the venue for the U.S. Open], the greens are so fast there and you have to have so much feel and touch. I felt like just going back to conventional was going to give me the best chance here and obviously there.

"You know, I thought about it for a while, and I was messing around on the putting green over the weekend. Just went back to the grip and really trying to focus on the basics like setup and eye position and ball position and stuff like that, alignment.

"One thing I did like about left hand low is it squared my shoulders up. So I'm really trying to focus on alignment and really setting up to the ball correctly each and every time and being really strict with that.

"I feel like, if I can put good structures like that in place, there's no reason for me to go with anything but what I've sort of had before."

McIlroy was encouraged by his performance on the greens on Thursday, as he took 29 putts at Muirfield Village.

"The margins in putting are so fine ... and I feel like the difference between having a good putting day and a bad putting day is very, very small," he added.

"It's sort of a mental thing for me as well. But I feel like I'm definitely on the right track and I holed some really nice putts out there and I got some good feeling. Hopefully, I can continue to stick with it and see improvement."

Dustin Johnson claimed the early lead in the Memorial with a round of 64, while world number one Jason Day (66) also started strongly.