Both Müller and Halys had trouble with serve returns in the first two sets, the official BB&T Atlanta Open site said in its Wednesday Recap.

The two were forced into a tiebreaker at 6-6 with Halys claiming the second set. Müller, who’s having his career-best year, appeared to relax in the third and closed out the final set on an ace up the middle. The match lasted a little less than two hours.

In the quarterfinals, Müller will face American Tommy Paul on Friday at 4pm Luxembourg time.

Earlier this month, Müller made it into the quarterfinals at Wimbleton, where he lost to Croatia’s Marin Čilić.