ERIN, Wis. -- As it turned out, Kevin Na versus Rory McIlroy was a mismatch of Conor McGregor-vs.-Floyd Mayweather proportions. But McIlroy, the four-time major champ, was the one who came away from Erin Hills on Thursday looking like a hopeless amateur lost in a foreign and hostile ring.

If you're a U.S. Open golf fan, you know that Na posted an Instagram video Monday declaring the fescue rough in this sprawling hayfield as more oppressive than the grasses in a Brazilian rain forest. And you also know that McIlroy came back the next day with a 1-iron stinger that had Na's name on it, even though he didn't mention him by name.

McIlroy was asked in his pretournament news conference about the USGA's decision to mow the fescue -- coincidentally, the governing body swore, after Na posted his mocking video. Rory responded as if he'd just been told that Europe had to give back the 2012 Ryder Cup.

"Really?" he said before wincing, rolling his eyes and shaking his head. "We have 60 yards from left line to right line. You've got 156 of the best players in the world here. If we can't hit it within that avenue, you might as well pack your bags and go home."

Na responded with a right hook to the Irishman's jaw, and the equivalent of a first-round knockout. He sent only one ball into the fescue all day and shot a 4-under-par 68. McIlroy put two balls into the fescue on No. 17 alone and shot a 6-over-par 78.

Ten strokes separated the men, as did one indisputable truth: Na has a far better chance of securing his first major victory than McIlroy does of securing his fifth.

"A lot of guys were supporting me out here," Na said of his fellow players. "A lot of guys were thanking me because they mowed the fescue because of me, getting a good laugh. They said, 'Hey, can you tell them the course is too long? They'll move up the tees.' The players get it, and I had a good time."

Before he teed off, Na had absorbed a pounding on social media from fans who called him a whiner (unfair) and who ripped him for being an excruciatingly slow player to begin with (fair). The Erin Hills design company piled on by hitting him with this tweet:

"Dear Kevin, Don't hit in the rough. Problem solved. Sincerely, Planet Earth. P.S. Fairway width @ErinHillsGolf is 2-3x most @PGATour stops."

"I don't look at that stuff," Na said of the social media blitz. "I looked at the first few hours, and then I gave up."

He was shaken by none of it. Na started on the back nine and birdied three of the final four holes before the turn. He pushed his score to 5 under, 2 strokes off Rickie Fowler's absurd pace, before finally finding the fescue on his 16th hole of the day.

"I hit it literally a yard into the fescue," Na said, "and I was lucky to find it. I want to thank the volunteers for spotting my ball and finding it."

He took an unplayable lie penalty stroke, accepted his bogey, and then responded with a par-saving putt at No. 8 and a birdie at the par-3 ninth to finish 3 strokes behind 18-hole leader Fowler. And then the questions came about the Sunday video that showed Na throwing a ball into the deep fescue, hacking away at it in vain and telling viewers that the players had to deal with such conditions on every hole.

"A lot of guys took my social media post kind of the wrong way," Na said. "If you read my post it says, 'I love the design.' I was just trying to show what we have in some spots. The fairways are generous, and I said all that. But most people don't like to read."

Na's caddie, Kenny Harms, said that his player put seven balls into the fescue in Sunday's practice round and then rediscovered his swing in a session on the range late Tuesday. Harms maintained that Na never intended to criticize the USGA or the setup of the course. Though Na plays slowly enough to make Jordan Spieth and Jason Day appear faster than Usain Bolt, his caddie said Na's concerns centered around pace of play and the potential for injury.

"But I do think these fairways are way too wide for a U.S. Open," Harms said.

McIlroy felt the same way, at least until he hit the back nine Thursday. "I hit the fairway on the 10th," McIlroy said, "and I didn't hit one on the way in."

He didn't hit Na, either. Didn't land a single punch. Let's see if his countryman, McGregor, can do a little better than that in August.