De Minaur looked set for a routine win, when he led by a set and 3-0, but Koepfer found a way back for a 3-6, 6-3, 7-6(6) victory in two hours and 33 minutes. Koepfer, who is making his ATP Masters 1000 tournament debut at the Foro Italico this week, will now challenge fifth seed Gael Monfils in the second round.
Koepfer won seven straight games before De Minaur broke in the third game of the decider, one of 13 service breaks in the match. De Minaur held a match point at 5-4, with Koepfer serving at 30/40, but the Australian overhit a forehand. In the next game, a poor forehand volley on break point cost De Minaur, and Koepfer then held to 15 for a 6-5 lead. The World No. 97 kept fighting and held his nerve in the tie-break.