Andrey Rublev was a surprise first-round loser at the Halle Open, while numerous seeds fell on the first day of the Queen's Club Championships.
Rublev came unstuck against Georgian Nikoloz Basilashvili, succumbing to a 7-6 (7-1) 6-4 loss as the world number eight crashed out in Germany.
Laslo Djere progressed after Henri Squire retired when 4-3 down, while sixth seed Pablo Carreno Busta secured a 6-3 7-6 (7-5) over French Open quarter-finalist Holger Rune.
Ugo Humber got the better of a topsy-turvy affair against qualifier Radu Albot, eventually triumphing 6-7 (4-7) 7-6 (7-4) 7-5 and Mackenzie McDonald defeated Marc-Andrea Huesler in straight sets.
Fourth favourite Taylor Fritz went down 6-3 6-3 against British wildcard Jack Draper, his first career win against a top-20 ranked player.
Reilly Opelka, the eighth seed, crashed out in the first round to New Zealand's Alex de Minaur, who claimed a straightforward 6-4 6-4 win.
Grigor Dimitrov, who won the event in 2014, battled past third seed Cameron Norrie 6-7 (2-7) 6-1 6-4, while Spaniard Alejandro Davidovich Fokina eased past compatriot Albert Ramos-Vinolas 7-5 6-4.
Alexander Bublik was 6-3 up before Italian Lorenzo Musetti retired, while seventh seed Marin Cilic capped the day's play in London by beating home hope Liam Broady 6-1 4-6 7-5.