Scheffler defending lead as McIlroy powers into British Open contention
World number one Scheffler is one stroke ahead of Matthew Fitzpatrick as he bids for a first Claret Jug, which would complete the third leg of a possible career Grand Slam, on 10-under par for the tournament.
Home favourite McIlroy, starting the day seven strokes off the pace, quickly made his move with three birdies in his first four holes.
A huge roar greeted a curling, 36-foot birdie putt on the first green, before the Northern Irishman tapped in for another birdie on the second.
A towering iron shot into the fourth continued the momentum, dragging the Masters champion to within four shots off Scheffler before the leader had even teed off.
Fitzpatrick, on nine-under overall, is a shot ahead of 2023 champion Brian Harman and China's Li Haotong, who make up the penultimate group.
Other early movers on Saturday included American Russell Henley, who was six-under par through 14 holes and also minus six overall.
Last year's runner-up Justin Rose, who lost to McIlroy in a play-off as the Masters in April, surged into the mix as well with four birdies in the space of five holes to get to six-under.
Reigning champion Xander Schauffele is six strokes behind Scheffler after his front nine, after an eagle on the par-five seventh kept him just about in touch.
Tommy Fleetwood, who finished second to Shane Lowry the last time the tournament was played at Portrush, was fighting to keep his hopes alive at four-under early into his back nine.
Scheffler will still be a strong favourite to close out his fourth major victory.
The American arrived at Portrush following a run of 10 successive top-10 finishes, featuring three wins including the PGA Championship.
Fitzpatrick is hoping to become the first English winner of the British Open since Nick Faldo won his third title in 1992.
He described a missed cut at the Players Championship earlier this year as the lowest point of his career, but has found some form in recent weeks.
The 2022 US Open champion carded a 66 on Friday, rolling in a 23-foot par putt on the 18th.
Harman is targeting a second British Open in three years following his six-shot triumph at Hoylake in 2023.
Li, who finished third on debut at the 2017 British Open, is still in the hunt to become the first Chinese man to lift a major title.