Altmaier earned the biggest win of his career by ranking to set up a second-round meeting with 86th-ranked Czech player Vit Kopriva.
The 26-year-old broke Fritz’s serve three times as he reeled off the final six games of the match, sealing victory in two hours and 41 minutes.
Fritz reached the French Open fourth round in 2024 but has lost four of his seven matches on clay this year.
Women’s ninth seed Emma Navarro also suffered a shock exit on Monday, going out in just 57 minutes as she suffered a 6-0 6-1 first-round thrashing by Spain’s Jessica Bouzas Maneiro.
Men’s defending champion Carlos Alcaraz made serene progress, beating Italian qualifier Giulio Zeppieri 6-3 6-4 6-2.
Two-time runner-up Casper Ruud avoided any scares, breezing past Spanish qualifier Albert Ramos-Vinolas 6-3 6-4 6-2.
Stefanos Tsitsipas, the beaten finalist in Paris in 2021, also advanced, beating Argentine Tomas Martin Etcheverry 7-5 6-3 6-4.
