British number two Cameron Norrie struggled to find his rhythm as he lost in straight sets to Monaco's Valentin Vacherot at the Paris Masters.
Norrie, 30, described his victory against world number one Carlos Alcaraz in the previous round as "the biggest win of my career" but was unable to reach that level again, losing 7-6 (7-4) 6-4.
Vacherot, who beat his cousin Arthur Rinderknech to win his first ATP title at the Shanghai Masters earlier this month, will face either Felix Auger-Aliassime or Daniel Altmaier in the quarter-finals.
"It was a really big performance," Vacherot said of his win over Norrie.
"I'm happy with how I handled the hot moments.
"Everything is clicking now. It's the work of many years - pure work."
The 26-year-old was ranked world number 204 in September but entered this week's tournament at 40th.
The Monegasque has won 10 of his 12 meetings with top-50 ranked players this year, with world number 31 Norrie the latest to fall.
Norrie dropped just two points on serve in the opening set but failed to capitalise on his one break point opportunity and eventually came up short in the tie-break.
The Briton was booed by the crowd in the French capital when he tried to get them on board by raising his arms in the second set.
He went on to pass up a further two break points at 2-2 and then dropped serve in the following game.
With momentum on his side and the finishing line in sight, Vacherot saved another two break points to lead 5-3 before cupping his hand to his ear and waving his racquet to the crowd.
He wrapped up victory on his second match point when Norrie hooked a forehand into the base of the net.
Those who still thought Vacherot winning Shanghai was somehow a fluke must now be reassessing their view.
Continuing to play with supreme confidence and clarity in the pressure points, Vacherot is proving any lingering cynics wrong in Paris.
Stringing together 10 match wins at the Masters events - the ATP's most prestigious tier of tournaments beneath the Grand Slams - is no mean feat.
Battling past Norrie, who won more points in the match but crucially not the pivotal ones, sees Vacherot become only the second men's player ranked outside the top 20 to put together such a run. Argentina's David Nalbandian was the other in 2007.
Vacherot's hefty serve is a clear weapon and allows him to alleviate pressure at crucial junctures.
But the 6ft 4in right-hander is far from just a 'serve-bot'. His fearless refusal to deviate from his risk-reward game, with his forehand as clutch as his serve, continues to pay off.
Vacherot also showed against Norrie - one of the tour's finest athletes - how well he can move behind the baseline.
Reaching the quarter-finals lifts him into the world's top 30 in the live rankings. It is a stunning and surprise rise, putting him on course to be seeded at the Australian Open in January.