A pre-tournament favorite, the Northern Irishman was an 8/1 shot when he walked to the first tee on Thursday. By the time he left for the second hole he had rapidly turned into a 33/1 outsider courtesy of a quadruple-bogey.

The four-time major winner carded an 8-over-par 79 and faces an uphill task to make the cut on Friday. His hopes of securing a second Claret Jug have all but disappeared and, according to figures compiled by VegasInsider, he is now a 250/1 shot.

The picture is even worse for Tiger Woods, who is a 500/1 outsider after finishing one shot ahead of McIlroy.

Koepka, meanwhile, started as second favorite at 10/1. Odds have now shortened to 7/2 after the world number one carded a 3-under-par 68 on Thursday.

The Floridian recorded four birdies in the first round and did not drop a shot until the 17th hole. Koepka has now finished in the top six in 14 of the last 17 rounds of a major and his form is ominous for the rest of the field, considering he has won four of the last nine majors.

“I feel like I played pretty solid,” he was quoted as saying by ESPN.

“I missed it in the right spots all day. One bad one on 17. That’s going to cost you. Didn’t really make any putts. Didn’t take advantage of anything to really go low. But definitely didn’t shoot myself out of it, so I’m OK with that.”

Behind Koepka, Jon Rahm is a 6/1 favorite to win at Portrush. The Spaniard was considered one of the players to watch this week after winning the Irish Open earlier this month and did his chances no harm at all as he shot 3-under-par in the first round.

Along with Koepka and Rahm, Tommy Fleetwood also finished 3-under for the day and is a 10/1 shot to win a first major on Sunday.

Fellow Englishman Justin Rose finished a shot adrift of Fleetwood but is expected to be in the mix and is currently 14/1, while Shane Lowry is a 16/1 shot.

The Irishman finished the first round in second place after posting a 4-under-par 67,

“I think four under is a great score on that course and I’ll take it any day,” Lowry told The Guardian. “I was probably as nervous as I’ve been in quite a while on the 1st tee, almost ever, I’d say.”

The 32-year-old is just one shot adrift of clubhouse leader JB Holmes.

The American played brilliantly on Thursday but remains a 30/1 outsider, alongside the likes of Sergio Garcia, 2016 Open winner Henrik Stenson and Jordan Spieth.

Holmes read the conditions better than anyone else and he hinted a controlled style of play might be key to success this weekend.

“You just have to accept the conditions over here and not get too greedy,” he said.

Updated odds

Brooks Koepka 7/2Jon Rahm 6/1Tommy Fleetwood 10/1Justin Rose 14/1Shane Lowry 16/1Patrick Cantalay 25/1Tony Finau 25/1Webb Simpson 25/1Jordan Spieth 30/1Sergio Garcia 30/1Henrik Stenson 30/1Tyrrell Hatton 30/1J. B. Holmes 30/1