Jung eliminated the third-seeded Opelka 5-7, 6-4, 6-4 and will face Andreas Seppi, who rallied in the second set and then rolled in the third for a 6-7 (2), 6-4, 6-1 victory over Jordan Thompson.
Jung won only two matches on tour last year—both in this tournament—and then qualified for the main draw this week. The 30-year-old Jung then ousted 2018 champion Kevin Anderson in his first-round match.
Opelka, who won his lone career title last year on Long Island, was the highest-seeded player remaining.
That's now No. 6 Miomir Kecmanovic, who came back to beat fourth-seeded Ugo Humbert 3-6, 6-2, 6-4. He will play No. 8 seed Kyle Edmund, who edged Soonwoo Kwon 3-6, 6-2, 7-6 (5).
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