Fils saved the only break point he faced while breaking Ruud's serve four times in a stunningly simple, 78-minute win on clay, Ruud's preferred surface. Fils won a whopping 28 of his 33 first-service points (84.8 percent).
In the semifinals, Fils will face German fourth seed German Alexander Zverev (26), who took care of France's Luca Van Assche (19) 6-3, 6-4.
Laslo Djere (28) of Serbia outlasted Italian third seed Lorenzo Musetti (21) 7-5, 6-3 to advance to the other semifinal match, where he will face China's Zhizhen Zhang (26), a 6-4, 6-4 winner over Germany's Daniel Altmaier (24).
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Plava Laguna Croatia Open Umag
Seventh seed Matteo Arnaldi (22) of Italy rallied past top-seeded Jiri Lehecka (21) of the Czech Republic 3-6, 6-2, 6-4 in a back-and-forth quarterfinal match in Croatia.
Lehecka held serve in the third set until the seventh game, when Arnaldi took control. Arnaldi hit eight aces and won 30 of 40 first-service points (75 percent).
He'll face Australia's Alexei Popyrin (23) in the semifinals. Popyrin earned a narrow 7-6 (2), 7-5 win over Croatian wild card Dino Prizmic (17).
The other semifinal match will pit No. 2 seed Lorenzo Sonego (28) of Italy against No. 6 Stan Wawrinka (38) of Switzerland. Sonego stormed back to beat Spain's Jaume Munar (26) 3-6, 6-1, 6-2, while Wawrinka polished off a 6-4, 7-5 win over Spaniard Roberto Carballes Baena (30).
Atlanta Open
Top-seeded Taylor Fritz (25) advanced to the semifinals but second-seeded Alex de Minaur (24) was upset at Atlanta.
Fritz cruised past Japan's Kei Nishikori (33) 6-4, 6-2, with the American saving the only break point he faced and posting a 7-1 edge in aces. His semifinal foe will be eighth-seeded countryman J.J. Wolf (24), who crushed Germany's Dominik Koepfer (29) 6-2, 6-3.
Seventh-seeded Ugo Humbert (25) of France downed the Australian de Minaur 7-6 (4), 6-3 in a match that ended after 1 a.m. ET. Next up for Humbert will be another Australian, Aleksandar Vukic (27), who earned a 6-4, 6-4 win over fourth-seeded Christopher Eubanks (27), an Atlanta native.