Denis Istomin produced an upset to defeat two-time runner-up Philipp Kohlschreiber on a rain-hit day at the Mercedes Cup in Stuttgart.
Only three of the scheduled matches were able to be completed, with fifth-seeded Kohlschreiber the biggest scalp.
Istomin, ranked 100, defeated the world number 22 German in straight sets, winning both in a tie-break 7-6 (7/2) 7-6 (7/3).
Elsewhere, Marton Fucsovics and Frenchman Gilles Simon also progressed before Feliciano Lopez saw his match against Viktor Troicki called off, with the Spaniard having taken the first set 6-1.
Wednesday will now see defending champion Lucas Pouille, in action, while Roger Federer, returning to action for the first time since early April following his decision to skip the clay-court season, takes on Mischa Zverev.
At the Libema Open in s-Hertogenbosch, fifth-seeded Greek teenager Stefanos Tsitsipas needed three sets to defeat lucky loser John-Patrick Smith of Australia 3-6 6-3 6-4.
However, number eight seed Andreas Seppi is out after the Italian lost 6-4 6-2 against American wild card Mackenzie McDonald.
Elsewhere, Australian Bernard Tomic came through his first-round match against Kevin King 6-4 6-2, while his compatriot qualifier Alex Bolt beat Vasek Pospisil of Canada in three sets.