There are over 70 golfers left to finish their second rounds on Saturday morning, but Horschel and Mickelson will likely share the last group when the third round gets under way.
According to Brian Wacker of PGA Tour, Horschel became the first US Open golfer since the USGA started keeping the stat in 1992 to hit all 18 greens in regulation for a round.
He shot a 67 in the process.
What makes his 3-under score even more impressive is that nobody else in his wave of golfers (78 in all) broke par for the round.
Horschel has been hot all year. He finished T2 at the Houston Open, T3 at the Texas Open, and T9 at the Heritage before winning the Zurich Classic in New Orleans.
No matter what he does this weekend, it will mark Horschel's best finish at a US Open (his previous best was getting cut in 2006).
Horschel should be in one of the final few groups on Saturday when the leaders tee off. High scores are flying around all over the place at Merion -- heck, Horschel might even be the outright leader.
No matter what happens, though, I promise nobody will ever break his greens-in-regulation record.