Munich - Defending champion Alexander Zverev had to dig deep to win his second-round match at the ATP tournament in Munich on Wednesday after a scare against fellow German Yannick Hanfmann.
The 26-year-old Hanfmann, ranked 118th in the world, took the first set, which lasted an hour, in a marathon tie-break before Zverev clawed his way back for a 6-7 (12/14), 6-4, 6-2 victory.
The 21-year-old Zverev, ranked third in the world, will now face his Davis Cup team-mate Jan-Lennard Struff in Friday's quarter-finals.
A potential opponent in the last four is Australian Open semi-finalist Chung Hyeon, who cruised into the quarter-finals with a 6-1, 6-1 win over Germany's Matthias Bachinger.
Chung, 21, lost only three first-service points, needing less than an hour to record his 19th win this season.
The South Korean had no problems, saving all five break points he faced and converting five of his seven break opportunities.
In the quarter-finals, Chung will play Martin Klizan, the 2014 Munich champion.
The Slovakian qualifier recovered from 3-0 down in the deciding set to beat Bosnian Mirza Basic 4-6, 6-3, 7-5 in just over two hours in his second-round tie.