Digimon Adventure V-Tamer 01.
It opens with Taichi, the main character of the series, trying to enter a gig with his V-pet, only to be disqualified.
Its because the digimon on his V-pet could not be identified and therefore was not a Digimon that was to be programmed into the V-pet.
However, after the tournament is over, Taichi plays with the tournament winner, Neo Saiba, and their fight ends in a draw.
Shortly thereafter, Taichi finds himself in the Digital World and meets his Digimon, Zeromaru, or Zero for a short time.