Padraig Harrington and Rory McIlroy both had opening rounds to forget at the Genesis Invitational in California.

Harrington, who is competing at the event after completing 10 days of self-isolation following a positive Covid-19 test, showed the effects of sitting in a hotel room for almost two weeks and struggled to get into his rhythm.

The European Ryder Cup captain started well enough with a birdie on his second hole but a run of five bogeys to the turn did for his round. He eventually signed for a 75 and is 11 shots off leader Sam Burns.

McIlroy is two shots better off than Harrington on +2 but will be disappointed with the way his opening round went.

Burns opened with an eagle and closed with three consecutive birdies while building a two-shot lead.

He shot a seven-under-par 64 at Riviera Country Club. Max Homa and England's Matthew Fitzpatrick are tied for second at five under.

It is the first time Burns has held the lead after the first round of a tour event, but he has been among the top five after the first round on four occasions this season - tied for the most of any player.

Burns, 24, is winless in 69 previous PGA Tour starts. His best result was a tie for third place in the 2018 Sanderson Farms Championship.

He reached the green on the 509-yard first hole in two shots, then sank a19-foot putt for eagle. After adding four birdies and two bogeys over the next14 holes, Burns sank birdie putts of 20, 10 and 4 feet to cap his day.

"I think we were just pretty consistent throughout the round," Burns said. "We knew we were going to have a tough stretch there to kind of start the back nine. I think just being patient out here (helped).

"This golf course at times doesn't offer you a lot of opportunities, so I think just with that mindset going in we were just going to kind of take what we could get."

Defending champion Adam Scott of Australia is tied for 19th at 2 under.