Phil Mickelson got off to a strong start as well in his bid for the career Grand Slam, attacking the course with deft iron shots on the way to shooting 70.
McDowell, who won the U.S. Open at Pebble Beach in 2010, was as steady as can be in the sand hills of North Carolina. The Northern Irishman bounced back from his only bogey at No. 4 with an eagle-3 at the par-five fifth hole. He added another birdie at the 14th and the rest of his card was filled in with pars, just the sort of solid, mistake-free golf that is required at a major championship won the last two years with above-par scores.
"You don't have to strike it amazing around here," McDowell said. "You just have to position the ball correctly at all times."
Na also made an eagle at No. 5 on the way to the best Open round of his career. He missed the cut in both 2010 and 2011, and finished 9 over at his last Open two years ago.
"There was some moisture on the greens and you were able to hold shots," Na said. "I was able to capitalize on a good tee time. But there's a long way to go. Obviously, I'm 2-under par right now, but at the end of the tournament even par is going to win this championship."
Spieth was in the thick of things again, making four birdies to put himself in contention at another major championship. He was tied for the lead heading to the final round of both the Masters and The Players Championship, but couldn't close out either on Sunday. It seems just a matter of time before the young Texan claims a career-defining triumph.
Maybe it will be at Pinehurst.
"I had a lot of fun today. You don't normally say that at the U.S. Open," Spieth said. "I was able to get into the flow early, and able to keep it going. One-under I would take that four times."
Matt Kuchar, who has never won a major title, shot 3-under 32 on the front side but a shaky finish knocked him out of the lead. He bogeyed the 16th and 17th before his par putt at the final hole tumbled into the cup on its last turn, bringing out a smile as he headed toward the clubhouse with a 69.
Also at 69 were Brandt Snedeker, Brendon Todd, Sweden's Henrik Stenson and Japan's Hideki Matsuyama.
Snedeker made Pinehurst look like the local muni on the front side, making the turn with a 31. The course bit back on the back side; Snedeker bogeyed the 10th, took a double-bogey at No. 11, and fell back to even par on the day with another bogey at the 12th. Snedeker pulled it together, sinking a couple birdies coming back to the clubhouse to put himself solidly in contention for his first major title.
Mickelson already has five majors, but this is the one he wants more than any other. Lefty has been the runner-up a record six times in this event, denying him the only big title missing from his resume. He's changed his grip to deal with a shaky putter, hoping that would help bring his first victory since capturing the British Open last July.
"This is a golf course where I get similar feeling to Augusta," Mickelson said. "You don't have to be perfect. You always have a chance. It is challenging. There are difficult shots. But they're manageable."
In recent weeks, Mickelson has been linked to an insider trading investigation. Clearly, he was able to stay focused on the course.
"I haven't done anything wrong," he said. "I'm willing to help out and would love to help out any way I can with the investigation."
Rory McIlroy, who won the 2011 U.S. Open in a rout at Congressional, opened with a 71. Defending champion Justin Rose, who held off Mickelson a year ago at Merion, shot 72.
The sun began to break through the clouds around lunchtime, with temperatures climbing to the upper 80s. That figured to firm up the greens and make things tougher for the afternoon players, including the world's top-ranked golfer, Adam Scott, and Masters champion Bubba Watson.