Germany's world No 192 Florian Mayer defeated teenage compatriot Alexander Zverev 6-2 5-7 6-3 to win the Gerry Weber grass court title in Halle on Sunday.
The 32-year-old is the lowest-ranked champion in the 24-year history of the Wimbledon warm-up event.
It is Mayer's second career title after Bucharest in 2013 and was clinched on a fifth match point.
Zverev, 19 and ranked 38 in the world, had knocked out defending champion Roger Federer in the semi-finals on Saturday.
The key to the final proved to be Mayer breaking to love after reeling off 11 straight points for a 5-2 lead.
The German would save a break point when serving for the match before closing it out on his fifth championship point after one hour and 55 minutes.
Mayer, who stunned world No 7 Dominic Thiem in the semi-finals, joins Tommy Haas (2009, 2012), Philipp Kohlschreiber (2011), David Prinosil (2000), Nicolas Kiefer (1999) and Michael Stich (1994) as German champions on the Halle grass.
Zverev was bidding for his maiden career title in just his second final, following defeat on the clay of Nice last month to Thiem.