Brooks Koepka
I'm not even sure he did, his reaction seemed like another ho-hum day at the gym after holing the winning putt on 18 and knowing no one was gonna catch him.
I suspected he was unsure if Harman could catch him by doing something amazing on 18, and Koepka didn't want to do a big "I just won the U.S. Open!" celebration when he wasn't completely certain that was the case. Not sure if you saw a couple minutes later in the cart ride to the scorer's office, but it seemed the magnitude of what he'd accomplished hit him then.