Rahm is eight shots behind Hovland, who enjoyed the lowest of his five career rounds in Harbour Town around the tight and layout.

The Norwegian was a stroke ahead of Brian Harman. Jimmy Walker and Aaron Rai were also at six-under but had not completed their rounds when play was suspended because of darkness. Six players will conclude their first rounds on Friday morning.

The group at 66 included US Open champion Matt Fitzpatrick, Ryder Cup captain Zach Johnson, Justin Rose, Sungjae Im, Scott Stallings and Joel Dahmen.

"That was really good," Hovland said. "Hit some really nice shots starting the day but didn't make the putts. It was a little frustrating, but stayed patient and just kept on hitting great shots. Then the putter started heating up, and it felt very stress-free from there on in."

Rahm, who slipped on his first green jacket just a few days earlier, finished at one-over 72. He put it down to the fatigue from Augusta National, and it didn't help his afternoon included a rain delay of nearly 90 minutes.

He never got comfortable on a course he had competed on only once before.

"I hate to make excuses, but a couple of the swings towards the end were my body being tired and surprised me," the World No.1 said.

"Hit some really nice shots starting the day but didn't make the putts. It was a little frustrating, but stayed patient ..." - Viktor Hovland.

The RBC Heritage is the sixth designated event this year on the PGA Tour, meaning an increased purse – up to $20 million from $8 million a year ago – has attracted the world's best like Rahm and Hovland who may have otherwise taken a post-Masters break.

Hovland birdied seven of his final 14 holes to also share the first-round lead at the Masters before finishing in a tie for seventh.

Hovland missed birdie putts inside 15 feet on three of his first four holes but didn't lose confidence in his stroke.

"Instead of freaking out or questioning all the reads that you make, that's not going to fix anything," he said. "So I just kind of kept trusting the process."

Adam Scott is the leading Australian at two-under.

His 69 leaves him tied 28th, one ahead of Cam Davis and two in front of Min Woo Lee.

After a week where PGA Tour and LIV Golf players were together, it was back to separate sides of the globe.

The focus now for the likes of Dustin Johnson, Marc Leishman and Cameron Smith will be on the next LIV Golf event on April 21-23 in Adelaide, Australia.