McIlroy pushed to solid British Open start by home support
The Northern Irishman was given a hero's welcome from his home crowd after ending his long wait to complete the career Grand Slam at the Masters in April.
McIlroy appeared to be on the charge when he reached three-under just after the turn, but a long day on the links took its toll as he made three bogeys in his last eight holes.
The world number two only managed to hit two of 14 fairways, but somehow still managed to card a 70 to leave him just three shots off the early lead.
"I feel the support of an entire country out there, which is a wonderful position to be in, but at the same time, you don't want to let them down," he told reporters. "So there's that little bit of added pressure."
It was a massive improvement from the opening 79 that ruined his tournament six years ago, when the British Open returned to Portrush after a 68-year absence.
"I felt like I dealt with it really well today," McIlroy added. "Certainly dealt with it better than I did six years ago. I was just happy to get off to a good start and get myself into the tournament."
The 36-year-old, who arrived earlier this week wearing his Masters green jacket, was undoubtedly the star attraction on a course where he fired a record-breaking 61 in 2005.
Nervy start
McIlroy found the left rough off the first tee, but avoided a repeat of his disaster on the same hole in 2019 when he went out of bounds en route to a quadruple-bogey.
A short missed par putt induced some groans from the thousands of fans around the green, but he quickly rebounded despite another wild tee shot with a birdie on the par-five second.
One house behind the fifth tee was adorned with a large "Go Rory" banner, while a group of children peered towards the fairway from a nearby road.
McIlroy gave them a birdie to cheer with an excellent curling putt.
He stayed otherwise bogey-free on the front nine, despite not finding a fairway until the eighth hole, almost holing his third shot on the par-five seventh after yet another wayward drive to the left.
McIlroy, who Bryson DeChambeau complained did not speak to him during the Masters final round, appeared in relaxed mood, chatting to playing partners Justin Thomas and Tommy Fleetwood during frequent hold-ups due to slow play up ahead.
He drained another putt to move to three-under after 10 holes, but could not recover after his latest drive into deep rough and gave the shot straight back.
McIlroy was in danger of throwing away his strong start when a tee shot into a bunker ultimately led to a bogey on the 12th, the easiest hole of the day, before another on 14.
Like McIlroy, the crowd started to run out of stamina as the clock passed 8:00 pm local time, with the number of fans lining the course dwindling.
Those still braving the increasingly chilly conditions were given a brilliant recovery shot from wide left on the 17th to enjoy, setting up a crucial birdie.
Five hours and 50 minutes after launching his bid for a second Claret Jug, McIlroy was given another rousing reception after tapping in on the final green to complete a round which kept him in the title hunt.
"I'm surprised four-under is leading. I thought someone might have gone out there and shot six or seven today," he said.
"Only three back with 54 holes to go, I'm really happy with where I am."