Rory McIlroy overcame a lack of practice time — and a bit of a hangover — to card a clean 4-under 66 in the opening round of the RBC Canadian Open at Hamilton Golf Club in Ontario on Thursday.

“A bit of a slow start. (Caddie Harry Diamond's) birthday dinner last night so had a couple of glasses of wine. I was probably feeling a little groggy when I woke up,” McIlroy admitted to GolfWeek after his round.

McIlroy teed off at 7:40 a.m. ET.

Rory also acknowledged some rust after a week of traveling.

“I certainly switched off,” McIlroy said in his pre-tournament press conference. “I went to one of my best friend’s wedding in Italy for four days … It was actually a really good trip, I needed it. Then I had a lovely, I had a great weekend at home. So I needed that reset. I’m playing four weeks in a row here … I’m sort of easing my way back into it. I probably hit a grand total of probably 150 balls last week.”

McIlroy won back-to-back PGA Tour events — the Zurich Classic of New Orleans and Wells Fargo Championship — before finishing T12 at the PGA Championship. During that span, McIlroy also filed for divorce and dealt with policy board drama.

On Thursday, McIlroy — easily the top-ranked player in the RBC field — began on the back nine, which he played without practice reps.

“I played the back nine blind,” said McIlroy. “Hadn't seen the back nine. Only played the front nine in the pro-am. So I was sort of happy enough to get out of there in even par and not make a bogey.”

(Scottie Scheffler won the RBC Heritage in April without playing the back nine).

McIlroy, who partnered with Canadians Nick Taylor (defending champ) and Taylor Pendrith, opened his round with nine consecutive pars, despite driving his tee shot on the par-5 17th into a hospitality tent.

“Some guy yelled out, ‘You need a wrist band to get in here,'” McIlroy recounted. “So that was pretty funny.”

Rory found his groove on the front nine. McIlroy birdied three of his last four holes to place him firmly in contention heading into Round 2.

“Once I got that nine holes out of the way I knew there were some chances on that front side and played probably much better on that front side,” he said. “Four birdies there, no bogeys was a good day's work.”

McIlroy hit all but two greens in regulation.

“At the start of the year, or at least through February, March, and April, I was making some big numbers, and there was a lot of volatility on the scorecard,” he said. “But to play bogey-free rounds is really nice again.”

Davis Skinns leads the field at 8-under.

McIlroy won Canada's national open by seven strokes (including a final round 61 with two bogeys) the last time Hamilton hosted, in 2019. McIlroy also triumphed in 2022 at St. George's.

McIlroy's group tees off at 12:55 p.m. ET on Friday.