And then, late in a day when the weather warmed up but very few golfers did, he hit a wall.
For his final 15 holes, Donald recorded six bogeys - five on a six-hole stretch of Merion's front nine. He needed a birdie on his next-to-last hole, the par-3 ninth, to salvage a strange but serviceable 72.
Did the completion of the first round, followed so rapidly by the second 18, drain the Ben Hogan-size (5-foot-9, 160 pounds) Donald?
"No, I felt good energy-wise," he said. "I had to play 23 holes today, but I'm done now. Some of the other guys are going to have to get up early again [on Saturday] and finish. I think that's more disruption than what we had."
If you listened to Donald's post-round remarks, it sounded as if he was struggling to be guarded in his comments about Merion and the way the USGA has set it up.
"The par-5s . . .. I didn't play very well," he said. "The other bogeys I didn't feel like I made too many mistakes. . . . The pins today were a lot more tucked. They were tougher to get to. A few were on little hills or slopes. It's very difficult to make those putts when the ball is breaking so much."
Asked if the rough that has been punishing the field and which the USGA has allowed to grow thick and long seemed more drastic than in other U.S. Opens, Donald demurred.
"It seems the same kind of rough every year to me," he said. "I haven't noticed a difference."
He ought to have mentioned all those things to David Graham, who won the 1981 Open at Merion, when the two men chatted earlier this week.
"I asked him 'What's the secret?,' " Donald said. "And he said, 'Keep it in the short stuff.' Obviously, I think most of us know that. It's about doing it."
Should Donald do that and eventually decipher these baffling greens, he knows he has a chance to become the first Englishman since Tony Jacklin in 1970 to win the U.S. Open.
"I would love to be a couple better," he said. "But certainly I think come the end of round two I'm going to be in a good place."
Contact staff writer Frank Fitzpatrick at ffitzpatrick@phillynewscom. Follow on Twitter @philafitz.