Gael Monfils missed out on a place in the Belgrade Open quarter-finals as the second seed was stunned by qualifier Roberto Carballes Baena 6-1 3-6 7-6 (7-4).
World number 15 Monfils would have been expected to breeze through against Carballes Baena, whose lone ATP title came back in February 2018.
But Monfils was trounced in the first set and, despite coming back from a break down to win the second, he could not avoid a shock loss as Carballes Baena reversed a 4-1 deficit in the decisive tie-break on Wednesday.
Carballes Baena's triumph sets up a last-eight clash with eighth seed Federico Delbonis, who beat Thiago Monteiro 7-6 (8-6) 3-6 6-4 at the ATP 250 event in Serbia.
Dusan Lajovic also moved through to the quarters after easing past Jeremy Chardy 6-3 6-4, with Andrej Martin the fifth seed's opponent after upsetting third seed Nikoloz Basilashvili 6-1 4-6 7-6 (7-4).
Elsewhere, top seed Lorenzo Sonego suffered a second-round exit at the Emilia-Romagna Open as Sebastian Korda claimed a surprise win.
Korda entered the tournament with just one clay-court win to his name in 2021 but followed up a first-round success over Andreas Seppi with a 6-1 7-5 defeat of Sonego.
Yoshihito Nishioka is his next opponent after the Japanese saw off Lorenzo Musetti 6-3 6-2.
Jaume Munar and Richard Gasquet also progressed, as did Jan-Lennard Struff, Norbert Gombos, Marco Cecchinato and Tommy Paul.
