Jon Rahm started his round strong and ended it even better, closing eagle-birdie-birdie for a 7-under 65 and a two-shot lead in the Arnold Palmer Invitational.

Not even the brute test of Bay Hill was a match for golf's hottest player on Thursday.

"Amazing round of golf," he said. "I wish all of them were as enjoyable as this one."

That doesn't mean it was perfection by any means. Rahm, playing in the afternoon when the greens became a bit more crusty, opened with three straight birdies.

He held steady the rest of the way until his big burst at the end allowed him to zoom past Honda Classic winner Chris Kirk and Cameron Young.

He hit only eight fairways. He twice was blocked by trees, one time escaping with par with a 30-foot putt on the 15th hole.

But it was his finish that was most impressive.

Rahm hit 5-iron to 25 feet on the fringe at the par-5 16th and holed it for eagle. On the par-3 17th, he hammered a 7-iron that cleared the bunker and landed in just the right spot to roll out to two feet. And on the closing hole, he hit a soft 9-iron to a front pin that settled about six feet away.

Rahm had said earlier in the week he doesn't think he can be beaten when he is firing on all cylinders, a belief by most top players. He also said he couldn't think of a tournament where he played his absolute best.

"Go through the round and you'll see plenty of mistakes," Rahm said. "I just took advantage of minimising mistakes and converted a couple of situations into really good scores. But it can always be better.

"But it's the first day," he added. "Ask me that on Sunday if I keep playing like this, and I'll probably change my answer."

Bay Hill is bracing for the worst over the next few days, with the wind expected to be strong on a course that already is fast and firm.

Kirk is coming off an emotional win nearly eight years in the making. He carried that momentum to seven birdies for a 67 during the morning round. Young also had a 67 in morning conditions that might be as easy as Bay Hill gets all week.

They were joined by Kurt Kitayama, who had a chance to challenge Rahm until dropping his lone shot on his final hole at No.9.

The group at 68 included defending champion Scottie Scheffler, Jordan Spieth, Xander Schauffele and Patrick Cantlay, all among the top 20 in the world.

Adam Scott headlines a 10-player group on 69.

The former world No.1 began with a bogey but then added four birdies - three on the second nine - to stay in touch with the chasing pack.

Cam Davies and Jason Day are the next best-placed Australians, both posting 1-under 71s to be tied 35th.

But Rahm looks as tough as ever. He already has three PGA Tour wins this year, and has won five of his last nine tournaments worldwide.

Kirk won the Honda Classic in a playoff just four days ago, a monumental win for the 37-year-old from Georgia who stepped away from the PGA Tour to seek help for alcoholism and depression.

Bay Hill will be really tough if the predicted strong wind arrives, but for now the bigger concern might be how to stop Rahm.