Reed, who had a hole-in-one Thursday and opened with a 66, shot an even-par 70 in Friday's second round at Winged Foot to grab a one-stroke lead over Bryson DeChambeau.
Reed started on hole No. 10 and had two birdies and three bogeys over his first nine. He opened his last nine with a birdie on hole No. 1 and offset two bogeys with two birdies the rest of the way.
The second round of the tournament proved to be much different than the surprisingly soft first day, as only Hideki Matsuyama (69), Bubba Watson (69) and DeChambeau (68) shot below 70 on Friday. Nine other players shot an even-par 70, while the rest of the field struggled.
On Thursday, 21 players finished under par -- the most in an opening round of a U.S. Open at Winged Foot -- and 32 golfers were at least even.
Rafael Cabrera Bello and Harris English both shot 70s to enter the weekend in a third-place tie with Justin Thomas at 2-under par. Thomas, who led after the first round, carded a 3-over 73 to slip two spots.
Jason Kokrak (71) was a shot back of that group at 1-under par. Thomas Pieters (74), Xander Schauffele (72), Matthew Wolff (74), Brendon Todd (72) and Matsuyama were tied for seventh at even-par 140.
Meanwhile, Tiger Woods failed to make the cut at Winged Foot after another disappointing day. He followed up his first-round 73 with a 7-over 77 and will miss the third and fourth rounds at the U.S. Open for only the fourth time in his career.
"On this golf course it's imperative that you hit fairways, and I did not do that," Woods said. "The whole goal of entering an event is to win, and when I don't give myself that opportunity over the weekend, it doesn't feel good.
"... It was frustrating that I didn't drive the ball as well as I needed to. It's frustrating that I'm not going to be here for the weekend and be able to compete for this great championship. It feels like the way the golf course is changing -- is turning -- that anybody who makes the cut has the opportunity to win this championship. I didn't get myself that opportunity."