Gilles Simon survived a scare against wildcard Maximilian Marterer to reach the second round of the German Tennis Championships on Monday, while compatriot Benoit Paire saw off Dmitry Tursunov in straight sets.
The fourth and fifth seeds are on course to meet in the quarter-finals should they both make it that far, but Simon - winner in Hamburg in 2011 - needed two hours and three minutes to win 6-2 4-6 6-4, having trailed 2-0 in the decider.
Paire fired down 11 aces in beating Tursunov 6-3 6-2 in less than an hour, while Florian Mayer made a winning start to his home tournament against Marco Cecchinato.
There were also wins for fellow Germans Jan-Lennard Struff and Cedrik-Marcel Stebe - the latter a shock victory over Damir Dzumhur - but Daniel Altmaier was eliminated by Horacio Zeballos.
In the only other match of the day, Diego Schwartzman claimed a comfortable win against Andreas Haider-Maurer and will face Stebe in the next round.
At the Swiss Open, Yannick Hanfmann set up a second-round clash with defending champion Feliciano Lopez thanks to a 6-3 6-7 (3-7) 6-4 victory over Facundo Bagnis.
Norbert Gombos, Denis Istomin and Gleb Sakharov also progressed to the next round with straight-sets wins before rain prevented any further action in Gstaad.