MAMARONECK, N.Y. -- All that hype over how tough the U.S. Open can be at Winged Foot. All that history of so many scores over par.

Thursday brought a surprise: all those red numbers.

Justin Thomas led the way with six birdies and only one bogey from a bad lie in the bunker, finishing with a 25-foot birdie putt that he barely touched for a 5-under 65.

It was the lowest score in a U.S. Open at Winged Foot, which is hosting the Open for the sixth time dating to 1929. And it was worth only a one-shot lead over Patrick Reed, Thomas Pieters of Belgium and Matthew Wolff, the 21-year-old Californian making his U.S. Open debut.

Rory McIlroy, who has been struggling to get off to a decent start in the majors, opened with a 67 and tried to contain his disappointment it wasn't lower. He had a pair of three-putts that held him back, one on the 329-yard sixth where he hit driver onto the middle of the green and had to settle for par.

McIlroy was among those two shots behind on a scoreboard filled with red numbers, an unusual site for Winged Foot. In the previous five U.S. Opens, a total of 17 players were under par after the first round. On Thursday, there were 21.

No one believes it will stay that way. Still, this was the day to take advantage.

"It's helpful with three days left, but it's not even remotely close to being over," Thomas said of his best start in a U.S Open or any major. "As great of a round and fun as it was, it's over with now, and I need to get over it."

Thomas went into the week saying it was a "different kind of fun" to grind over pars, to hit middle irons to difficult pins instead of the low scoring at so many other tournaments.

This was fun, too.

Reed and Will Zalatoris each made a hole-in-one on No. 7, and Zalatoris somehow missed another ace on No. 13. Spanish amateur Eduard Rousand holed out for eagle from the first fairway with his second shot in his U.S. Open debut. Louis Oosthuizen holed out for eagle on second fairway.

"Yeah, 65 is fun no matter where you play, especially at Winged Foot," Thomas said. "I was in a really good frame of mind, and I was focused. I just was sticking to my routine and playing every shot, as opposed to getting ahead of myself. It's one of those rounds where ... next thing you know, you make the putt on 18, you're done for the day."

He played with Tiger Woods and PGA champion Collin Morikawa, who couldn't get done fast enough. Woods shot a 73, and Morikawa, the PGA champion, shot 40 on the back nine for a 76.

Pieters and Wolff dropped only one shot along the way while playing in the afternoon, when the greens became a little more difficult because of all the foot traffic. Even so, low scores -- at least by Winged Foot standards -- kept coming.

"There's a couple of guys that went a little lower than maybe expected," McIlroy said. "But it's not as if the rest of the field are finding it that easy."

Raise your hand, Phil Mickelson.

Returning to the scene of a double bogey on the 72nd hole that cost him the 2006 U.S. Open, Mickelson opened with two birdies and still struggled to break 80. He went rough-to-rough on the 18th and shot 79. Dustin Johnson, the No. 1 player in the world, couldn't make putts and shot 73.

Low clouds added to the soft feel of Winged Foot. The greens were fast but not as firm. Jordan Spieth discovered that on the first hole. He was trying to figure out if his approach would take a hard hop and release or land soft and spin back. He guessed wrong, the ball rolled down the slope to the front of the the green and he three-putted for bogey. He also shot 73.

Thomas judged it correctly with a short iron left of the flag that spun back close to the hole for birdie. His only dropped shot came at the par-3 third because the ball settled on a downward slope of a bunker, leaving him no chance to get it close. There were few complaints the rest of the way.

He also made three birdies around the turn, narrowly missing a fourth in a row from just inside 10 feet at the par-5 12th. Thomas felt at ease, and Winged Foot allowed him to feel that way. At least Thursday.

The previous low score for a U.S. Open was a 66 by Fuzzy Zoeller in the second round in 1984.

"The greens are very soft," Thomas said. "I thought they'd be a little firmer, but I also understood that they need to err on this side of they can get them how they want this weekend. ... It's still Winged Foot. You've still got to hit the shots."