Billy Horschel
Billy Horschel (pictured) and Phil Mickelson are the only players shooting under par in the U.S. Open at Merion GC. Reuters

Americans Phil Mickelson and Billy Horschel are the only players currently under par in the U.S. Open at Merion, but the field is still wide open as the second round wraps up and the third round begins Saturday.

Hitting every green in his second round, Horschel sunk four birdies and only slipped up on the 13th hole with a bogey for a 1-under 67. The 26-year-old is tied with Mickelson at the top of board, not falling prey to the smaller Merion course.

Like Horschel, Mickelson limited his mistakes in the second round and stayed on top with a 2-over 72. Overall he’s 1-under 139. Bogeys on the first, 12th, and 13th holes prevented Mickelson from building off his strong play in the first round, but he sunk a birdie putt on the 18th to keep up with Horschel.

Luke Donald, Steve Sticker, and Justin Rose are tied for third at even par.

Tiger Woods and Rory McIlroy both made par on the rain soaked course with a 70 in the second round, and are part of a seven-way tie for 13th with a 3-over 143. McIlroy notched four birdies, and Woods three.

Woods has been bothered by some pain in his left arm, and reportedly winced through several shots, but is still well within striking distance.

The field might welcome the first completely sunny afternoon of play, since the first and second rounds were suspended to due to severe thunderstorms in and around the greater Philadelphia area. The forecast for a high of 80 and mostly sunny should dry out the course.

The full leaderboard can be followed on Golf Channel here.

NBC’s coverage begins at noon Eastern time, or watch their live online stream at NBC Live Extra here.