Koepfer won 6-4, 6-4, 7-6 (2) over Reilly Opelka in the second round of the US Open on Wednesday.
The No. 118-ranked German is having the tournament of his life, as he came into this event with just two tour-level wins combined in the last two seasons, but after defeating Jaume Munar in four sets in the opening round, he followed that up with another upset of the big American. Koepfer, 25, is doing all this after qualifying for the main draw. In Round 3, he'll take on either Jenson Brooksby or No. 17 seed Nikoloz Basilashvili.