There were 68 players still on the Merion Golf Club course when darkness forced a stoppage of play, but the tournament came close to getting back on schedule after rains twice disrupted the action on Thursday.
A crowd of 16 players were within three shots of the lead when play was suspended and there was an outside chance that Mickelson and Horschel could be passed before the second round comes to a close early Saturday.
None of those 16 has ever won a U.S. Open, although Mickelson has finished second a record five times.
Mickelson began the second round with a one-shot lead and he wound up with a 2-over 72 -- a score that could have been much lower had he taken advantage of a series of birdie chances.
He finished 36 holes at 1-under 139. Horschel reached that same figure by putting together a 67 that equaled the low round of the tournament.
Horschel recorded his first PGA Tour win this year in New Orleans and his six Top 10 finishes in 2013 are tied for the most on the circuit. He is fifth on the tour's money list and on Friday became the first player in 20 years to hit all 18 greens in a round at the U.S. Open.
When darkness fell, Mickelson and Horschel had a one-shot lead over four of the world's best players as well as a junior at the University of Washington.
Luke Donald, Steve Stricker and Justin Rose all completed their second round at even-par 140 and Ian Poulter was even par with four holes to play. Also at even par was amateur Cheng-Tsung Pan, a member of the Washington golf team and a native of Taiwan.
Pan did not tee off until 5:42 p.m. EDT Friday and he will have nine holes left in his second round on Saturday.
Then came Tiger Woods and Rory McIlroy, part of a tie for 17th place at 2-over 142. Woods is seeking his first major championship in five years and McIlroy is attempting to win the U.S. Open for the second time in his last three tries.
Mickelson three-putted the opening hole, then battled his way to a string of pars that kept him in the lead. But he missed birdie opportunities at the second, eighth, ninth and 11th holes before suffering consecutive bogeys at the relatively easy 12th and 13th.
He avoided a major collapse, however, and came to the final hole, playing as the toughest on the course, trailing Horschel by a single shot.
Mickelson drove into the fairway at the 18th, put his second shot on the left edge of the green and then hit a perfect putt to grab a share of the lead.