Szisz's primary duties as the head of testing at Renault limited the number of races he could compete in. However, in 1906 he achieved a permanent place in the annals of auto racing when he and his riding mechanic M. Marteau drove a Renault AK 90CV to victory in the first Grand Prix race in Le Mans. He averaged . His victory in the French Grand Prix and the commercial success of the race soon led to the establishing of other Grand Prix races throughout Europe. The following year, Italy's Felice Nazzaro, who had finished second behind Szisz, captured the second French Grand Prix. Szisz competed in the 1908 race but did not finish and suffered a similar fate following mechanical problems in Savannah, Georgia at an American Grand Prize race organized by the Automobile Club of America.