He made his debut in the 2015 Moto3 World Championship, racing from the second round of the season onwards, after being replaced for the opening event by Marco Bezzecchi due to age limits.

[2] Riding on a Mahindra MGP3O entered by San Carlo Team Italia, alongside his compatriot Matteo Ferrari, he finished in the points four times, his best result being a twelfth place in Aragon.

In 2017, Manzi moved up to the Moto2 class with Sky Racing Team VR46 alongside fellow Italian Francesco Bagnaia, riding a Kalex.

Manzi had a bad year, finishing in the points only twice out of 15 races (a 10th place in France, and a 14th place in Austria), before being involved in a nasty crash during the race in Misano, where Romano Fenati reached over, and pulled Manzi's brakes at 200 km/h.

[3] Manzi managed to stay on the bike, but later crashed out on his own, while Fenati was black-flagged and disqualified from the race.