WATCH the ATP Tour LIVE in Australia on beIN SPORTS | Subscribe here and take advantage of our 7-day FREE TRIAL

Muller was rock-solid behind serve and went toe to toe with Zverev from the baseline to overcome the World No. 3 in a two-hour, 41-minute epic.

With his second-round triumph at the German clay-court ATP 500, Muller set a new personal-best tally for tour-level match wins in a single season (17). Muller will face Felix Auger-Aliassime in just his second quarter-final at ATP 500 level or above. The Canadian beat Giovanni Mpetshi Perricard 6-2, 6-4.

Earlier this season, Muller captured his maiden tour-level title in Hong Kong and is now up to No. 39 in the ATP live rankings. He previously reached a career-high No. 39 in April this year.

Earlier, third seed Andrey Rublev made a confident start to his Hamburg campaign with a 6-3, 7-5 triumph against home talent Justin Engel.

Rublev, the only other former champion along with Zverev to line up in the draw in northern Germany, dropped serve in the opening game but went on to convert four of 13 break points he earned against the 17 year-old Engel.

Now 13-11 for the 2025 season after winning consecutive matches for the first time since he lifted the Doha title in February, Rublev will take on Luciano Darderi in the quarter-finals as he looks to add a second Hamburg title to his 2020 triumph.

Jiri Lehecka and Tomas Martin Etcheverry set a last-eight meeting with comprehensive second-round victories. Lehecka downed fourth seed Francisco Cerundolo 7-5, 6-2.

While Etcheverry overcame his fellow Argentine Camilo Ugo Carabelli 6-4, 6-4.

Roberto Bautista Agut notched his first win against a Top 20 opponent in 30 attempts dating back to 2014 by downing second seed Frances Tiafoe 6-2, 6-3. 

The Spaniard will face Flavio Cobolli for a semi-final spot, after the Italian defeated fifth seed Alejandro Davidovich Fokina 6-4, 7-5.