Rickie Fowler says you can't win a tournament on Thursday, but you can take yourself out of contention. (0:38)

PEBBLE BEACH, Calif. -- Low scores, birdies and eagles typically leave the USGA unhappy at its championship events. The U.S. Open is supposed to be the most difficult test among the sport's four majors, and the one that challenges a player's complete game the most.

So what does the USGA, which has been heavily criticized for its over-challenging and perhaps unfair setups, do after its leaderboard at the U.S. Open at Pebble Beach was littered with red after Thursday's opening round?

On a cool, dry Northern California day, with little to no wind blowing off the Pacific Ocean, many of the world's best players had their way with one of the most iconic golf courses.

Justin Rose matched the lowest score in an opening round of a U.S. Open at Pebble Beach, shooting 6-under 65 to match what Tiger Woods posted in the first round in 2000. Four players -- Rickie Fowler, Xander Schauffele, Louis Oosthuizen and Aaron Wise -- sit one back at 5 under. That's more players that were 5 under or better after 18 holes of the previous five U.S. Opens at Pebble Beach combined.

There were 39 players under par in the first round, including 24 in the morning wave alone. That's second most in a U.S. Open in the past 10 years (there were 44 players under par after the first round at Erin Hills in 2017) and 30 more than the last time Pebble Beach hosted in 2010.

"I thought [the USGA] did a really good job," Oosthuizen said. "The greens were receptive. It was running nicely. I think the golf course was set up really good for good scoring in the morning and will probably get a little bumpy in the afternoon and firm up a little bit."

Unless conditions change dramatically, this U.S. Open figures to look a lot more like Brooks Koepka's win at Erin Hills in 2017 (when he finished 16-under) than his victory at Shinnecock Hills last year (when he was 1-over).

Still, players don't expect the USGA to sit idly by as players have their way with the par-71 track.

"Going into the weekend, we know what's going to happen with the U.S. Open," said Jon Rahm, who is 2-under after 18 holes. "Pins are going to get tricky, greens are going to get firm, and it's going to be difficult."

Fowler said he expected changes to be made, even after the USGA was heavily criticized for letting things get out of hand at Shinnecock Hills last year and at Chambers Bay in 2015.

"I'm assuming the golf course is going to continue to firm up over the week and it's going to get tougher to control the golf ball, and you have to be a little bit more dialed in," Fowler said.

Can Rory stay hot?

Rory McIlroy shot a 68 on Thursday as he looks to win for the second week in a row. Warren Little/Getty Images

No player has ever won the U.S. Open after winning a PGA Tour event the previous week.

Rory McIlroy, who won by seven shots at the RBC Canadian Open last week, is trying to change that.

McIlroy's opening 3-under 68 was his lowest round in a U.S. Open since firing a 66 in the final round at Chambers Bay in 2015. It's the first time he has opened a U.S. Open with a round in the 60s since 2011, when he had four straight for an eight-shot victory at Congressional.

It's also a nice turnaround after he started poorly at the Masters and PGA Championship earlier this season, as well as in the previous three U.S. Opens, when he missed the cut in each. In fact, he was over par in the first round of each of the previous seven U.S. Opens.

"I think it's important for everyone [to start fast], but especially for me in trying to get my way back to winning these big events," said McIlroy, who hasn't won a major since 2014. "It is important. The first two majors of the year, I shot over par. It's so hard to chase, especially on golf courses that are tough."

Will the champ make a move?

play 1:28 Koepka pleased going 2-under on Thursday Brooks Koepka says he hasn't been worried about coming back from injury and that he's pleased with his score in Round 1.

Koepka, the two-time defending U.S. Open champion who is trying to become the first player to win three in a row in 114 years, looked as if he was going to pick up where he left off at the PGA Championship a month ago.

Koepka birdied four of his first six holes, including one on the par-3 fifth, where he chipped in from the rough. But then Koepka had only one more birdie and had bogeys on Nos. 8, 13 and 17. He had to hit his second shot off the cart path on No. 18 to save par.

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He finished 2-under 69, four shots behind Rose. Koepka hit only 7 of 14 fairways and 12 of 18 greens in regulation.

"It's a battle if you're not going to hit fairways," he said. "If you're not going to hit greens, it's going to be tough. I'm actually quite pleased -- 2 under, I didn't shoot myself out of it. I'm right there. I feel like if I get off [Friday], get off to a good start, I'm right back into it."

Is Phil a factor?

Phil Mickelson, who is seeking his first U.S. Open victory in his 28th start to complete the career grand slam, didn't open the way he wanted.

Mickelson shot 1-over 72, with two birdies and three bogeys -- including one on the par-4 third hole, in which he missed a 22-inch putt.

Mickelson, who turns 49 on Sunday, says he believes he's still in the hunt with 54 holes to play, though.

"You stay around par at the U.S. Open, you'll be OK," Mickelson said. "And I just need to go out and shoot something in the 60s [on Friday] and I'll be in a good spot in the weekend. And I'm playing well enough to do it."

The bad news: The five-time major champion has never won a major when he shot over par in the first round.