Kevin Kisner was still the man to catch on the second morning of the 147th Open Championship but he had a stellar chasing pack breathing down his neck.
Exactly four years on from his Open victory at Royal Liverpool, Rory McIlroy fired two birdies in his first 11 holes on Friday to join the large group one shot behind American Kisner, who will begin his second round in the afternoon.
Zach Johnson, who lifted the Claret Jug in 2015, was also on four under after carding three birdies and a single bogey in his opening eight holes, alongside later starters Erik van Rooyen, Tony Finau and Zander Lombard.
Francesco Molinari, Thorbjørn Olesen, Haotong Li, Brandon Stone and Tommy Fleetwood were another stroke back on three under.
After opening his round with six straight pars as the morning rain continued to fall at Carnoustie, McIlroy carded his first birdie of the day at the seventh after hitting his approach to 12 feet.
The four-time Major Champion then sank another birdie putt from inside 15 feet at the tenth to get within a shot of the lead.
McIlroy had a good opportunity to join Kisner at the summit after landing his approach to the 11th 12 feet from the hole but he had to settle for a par.
It was far from a perfect start for Johnson, who dropped a shot at the first to fall to one under, but he made amends with back-to-back birdies at the third and fourth.
The 42 year old added another at the par-five sixth to get to four under.
