Vereniki Goneva ran in four tries for the Flying Fijians, while former soldier Leone Nakarawa and Naipolioni Nalaga also crossed and Seremaia Bai added 19 from the boot.
The Namibians, though, were more than competitive - with Theuns Kotze slotting over 15 points, including a hat-trick of drop goals, while Heinz Koll and Chrysander Botha scored wonderful tries.
Namibia led twice in the early stages but they just couldn't cope with the counter-attacking of the South Sea islanders.
A booming 55m penalty from Kotze had put the lowest-ranked team in the tournament into the lead after three minutes.
Fiji responded almost immediately when quick turnover ball created the space for Goneva to brush aside the challenge of Conrad Marais and score. Bai converted.
Namibia were back in front within 10 minutes, though, as the 24-year-old Kotze landed a three drop goals in quick succession.
Bai cut the deficit to two points with a penalty before Fiji retook the lead with two tries from counter-attacks.
First Nakarawa, who resigned his commission in the Army to play in the tournament, crossed in the corner - with Bai adding the extras.
The giant lock then turned provider two minutes later as he collected Waisea Luveniyali's chip ahead before shipping the ball to Goneva to score.
Bai missed the conversion but made amends with a penalty to put Fiji 25-12 in front.
Namibia attempted to claw their way back into the game with Danie Van Wyk twice going close before Kotze added his second penalty.
Goneva, though, completed his hat-trick on the stroke of half-time as he held off Raoul Larson to score in the corner. Bai converted to put Fiji 32-15 up at the break.
The try of the match came just after the interval as second row Koll eventually crashed over following a brilliant break by Van Wyk and support from Marais. From quick ball, Koll showed a lovely dummy in midfield before accelerating to the line for his second international try.
Namibia's joy was shortlived as former Sevens skipper Goneva finished off his fourth try after Gaby Lovobalavu's break. Bai added the extras.
Namibia weren't finished, though, as full-back Botha scored in the corner. The pack set the platform with a catch-and-drive from a lineout before good handling released the 24-year-old.
The Africans continued to press for another score, with Van Wyk, Botha and Marais all threatening, while Danie Dames was held up over the line.
And Fiji were able to see out the match as Bai slotted over a penalty and then converted a late try from Nalaga.
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