After rattling through the first set, the teenager picked up the pace in the second to surge into a 5-0 lead in less than 20 minutes.
A Raducanu double-fault in the sixth game gave Uchijima a whiff of survival and the Japanese won four straight games, before Raducanu finally sealed the match with a 114-kilometre per hour ace.
The match lasted one hour and 18 minutes.
Raducanu burst onto the scene last year when she made history by emerging from qualifying to claim a maiden Grand Slam title.
It was the first time a qualifier had won a major.