The 35-year-old Scot birdied five of the last six holes on his opening nine, then played the final nine in one under with a bogey on the par-4 2nd and birdies on the par-5 5th and par-4 6th.

Chile's Joaquin Niemann was a stroke back with Argentina's Emiliano Grillo and American Tom Hoge.

Knox has two victories on the PGA Tour and one on the European Tour. He lost in a playoff at Mayakoba in 2016.

"Obviously, I love the course. It sets up well for me. I mean, historically, my driving accuracy and iron play's been my strength, so that's kind of the key things that this course brings out in you," Knox said.

"Obviously, I love the course. It sets up well for me. I mean, historically, my driving accuracy and iron play's been my strength, so that's kind of the key things that this course brings out in you." – Russell Knox

Defending champion Brendon Todd was at 67 in a group that included Tony Finau, Viktor Hovland, Aaron Wise, Nate Lashley, Akshay Bhatia and Michael Gligic.

Former US Amateur champion Andy Ogletree, the low amateur at the Masters, had a 69 in his pro debut.

Ryder Cup captain Steve Stricker also shot 69.

Rickie Fowler had a wild round of 70 with eight birdies and quadruple and double bogeys. He played the final six holes in four under to get under par.

Four-time major champion Brooks Koepka had a 71.

Justin Thomas shot 72 and at No.3, he's the only player from the top 10 in the world in the field.

Aaron Baddeley and Cameron Percy also opened with rounds of 72 to be the best of a small Australian contingent.