Zverev overcame the former World No. 3 with 6-7(2), 7-6(8), 6-1, and 6-1. The German, who has advanced to the Round of 16 in each of his last four tournament appearances, will face sixth-seeded Jannik Sinner, who earlier in the day defeated 2016 US Open winner Stan Wawrinka in four sets.
The Swiss wild card overpowered from the baseline in a 6-3, 6-4 triumph against the 10th-seeded American, clinching an emphatic upset in a match that lasted for one hour and 18 minutes.
Wawrinka has been impressive in his first appearance in Umag since 2007 and has won all six sets played at the clay-court ATP 250 event. He aims to clinch his first tour-level title since Geneva in 2017.