(Reuters) - World number five Alexander Zverev of Germany breezed into the second round of the Halle Open with a 6-4 7-5 victory over Dutchman Robin Hasse on Monday.
Second-seeded Zverev struggled on serve at times and finished with seven double faults but he fired 12 aces and converted four break points to advance after an hour and 45 minutes.
Russian third seed Karen Khachanov also progressed by winning 7-6(5) 6-4 against Serbia's Miomir Kecmanovic -- a late replacement for Damir Dzumhur who withdrew from the tournament.
However, there was an early upset when fifth seed Gael Monfils lost 7-6(6) 6-4 to French compatriot Pierre-Hugues Herbert.
Top seed Roger Federer is in action on Tuesday against John Millman, who beat the Swiss at the U.S. Open last year.
(Reporting by Rohith Nair in Bengaluru; Editing by Ken Ferris)