"It's helpful with three days left, but it's not even remotely close to being over," Thomas said of his best start in any major. "As great of a round and fun as it was, it's over with now, and I need to get over it. ... Yeah, 65 is fun no matter where you play, especially at Winged Foot. I was in a really good frame of mind, and I was focused.
"I was just 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."
In total, 21 players finished the first round under par -- the most in an opening round of a U.S. Open at Winged Foot -- and 32 golfers were at least even.
"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."
Dustin Johnson -- the top-ranked player in the world -- failed to make his putts and shot 73. Tiger Woods, Jordan Spieth and Justin Rose also opened with 73s and had their share of struggles on the course.