French Open finalist Ruud upset by qualifier Struff in Monte Carlo
The 100th-ranked Struff - who came through the qualifiers - will play Russian Andrey Rublev in the quarter-finals after the latter beat his compatriot Karen Khachanov 7-6(4), 6-2.
Ruud was on a run of nine successive victories on clay - having won in Gstaad last July and then Estoril last Sunday - but he looked out of sorts in Monaco.
The world number four - who also reached the US Open final last year - saved two match points and battle back from 5-2 down to lead 6-5 in the second set but Struff levelled.
The German then took a hard-fought tie-break - Ruud saving a third match point - when the Norwegian sent a return out to reach the last eight.
Ruud was not the only Grand Slam losing finalist to exit in the last 16.
Matteo Berrettini, who lost to Novak Djokovic in the 2021 Wimbledon final, withdrew due to a muscular problem the Italian suffered in Wednesday's three-set marathon win over Francisco Cerundolo.
Berrettini's withdrawal allowed Denmark's ninth-ranked Holger Rune to move into the last eight of the tournament for the first time.
The 19-year-old will face the winner of the match between in-form Russian Daniil Medvedev and Germany's Alexander Zverev, who is still battling back to form after a long absence due to tearing ankle ligaments in the French Open semi-finals last year against Rafael Nadal.
Berrettini's compatriot Jannik Sinner enjoyed better fortune saving a match point before beating Poland's Hubert Hurkacz 3-6, 7-6(6), 6-1.
The 21-year-old Sinner is likely to face Serbian legend Djokovic, who plays another Italian, 16th-seeded Lorenzo Musetti later on Thursday.
"I needed such a match," said Sinner.
"I am now into the rhythm.
"I am not yet at my best level but each day I am making progress."
The 2021 French Open finalist Stefanos Tsitsipas breezed through his third-round match, the Greek swatting aside Chile's Nicolas Jarry 6-3, 6-4.
