Krajinovic rallied to beat Joao Sousa of Portugal 3-6, 6-4, 6-4 — with both players hitting six aces and dropping serve three times— while Khachanov had 15 aces in a 6-3, 7-6 (2) win against Ruben Bemelmans of Belgium.
Krajinovic next faces French veteran Gilles Simon, who edged out Maximilian Marterer of Germany 7-6 (3), 7-6 (4) as both players dropped serve twice.
In other first-round matches Tuesday, Belarusian qualifier Ilya Ivashka won 6-3, 6-1 against Laslo Djere of Serbia; Italian qualifier Stefano Travaglia beat Hugo Gaston of France 6-2, 6-2, and Frenchman Pierre-Hugues Herbert had nine aces in a 6-2, 7-6 (3) win against Slovakian qualifier Norbert Gombos.
Herbert next faces third-seeded countryman Lucas Pouille, who recently won the Open Sud de France title in Montpellier.
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