Frances Tiafoe shrugged off his first-round loss at the Sardegna Open with a successful hard-court return, beating sixth seed Dusan Lajovic at the European Open.
The American had lost to Corentin Moutet on the clay in Italy but was back to form in Antwerp in only his second meeting with the Serbian.
Lajovic won their previous encounter at Indian Wells in 2017 but Tiafoe saved two set points in the second set to prevail 6-3 7-6 (8-6) and progress to the second round.
Tiafoe was not the only player to knock out a seed on the ATP Tour on Monday, as his compatriot Steve Johnson claimed victory over Marin Cilic at the Cologne Championships.
Johnson spurned four set points as the opener was sent to a tie-break. However, he prevailed in the shoot-out and then broke the ninth seed to love in the first game of the second.
Cilic never threatened to break back as the 2014 US Open champion slipped to a surprise defeat.