Halle, June 18: Gael Monfils suffered a surprise early exit at the Halle Open, but Alexander Zverev safely booked his place in the second round on Monday (June 17).
Fifth seed Monfils was beaten at the opening stage 7-6 (8-6) 6-4 by French compatriot Pierre-Hugues Herbert, ranked 43rd in the world.
Germany's Zverev progressed on home soil with a 6-4 7-5 victory over Robin Haase in what was his first triumph of the year on grass after defeat to Dustin Brown in Stuttgart last week.
Third seed Karen Khachanov also sealed a spot in the last 16 with a 7-6 (7-5) 6-4 success at the expense of Miomir Kecmanovic.
There were also wins for Radu Albot, Steve Johnson and Portuguese qualifier Joao Sousa.
Top seed Roger Federer faces John Millman on Tuesday with the chance to gain revenge for his defeat to the Australian in last year's US Open.
The man who defeated the Swiss ace in the Halle final last year, Borna Coric, opens the defence of his title against Spain's Jaume Munar.