In 1932, he won a race at Rapallo against Moto Guzzi's star rider, Pietro Ghersi.

For the 1934 season, Moto Guzzi developed a new V twin 500 cc racer and Tenni rode it to victory at the Italian Grand Prix ahead of his teammate Stanley Woods.

It was here that he came to be dubbed the Black Devil referring both to the color of his hair and his diabolical riding style.

He suffered serious injuries in the 1938 and 1940 seasons then his racing career was put on hold by World War II.

In the mid-1930s Tenni ventured into car racing, and in 1936 and 1937 drove for Maserati, but was not as successful as on two wheels, although he did compete in the 1936 Mille Miglia, finishing first in his class and fifth overall.