Hubert Hurkacz fended off the on-form Koepfer in three sets, only looking in control in the last set.
Koepfer immediately went on the offensive, taking an early break in the first set and winning it 6-4.
The second set was a struggle, with both players trading breaks until Hurkacz took it 7-5.
In the final set, Hurkacz finally took control, winning with two breaks.
The match lasted just over two hours and the final score was 4-6, 7-5, 6-2
Hurkacz keeps his hopes of reaching the tour finals alive as he sits in eighth, Ruud and Norrie are also still in the running.
The Pole makes the fourth round, where he’ll face Duckworth.