First half tries from Vereniki Goneva, Rupeni Nasiga and Kini Murimurivalu and 12 points from the boot of fly-half Seremaia Bai powered the hosts to victory after suffering a 45-21 in the Pacific Nations Cup last month.
Rising star Viliami Longi grabbed two tries for Tonga to underline his World Cup credentials but it was not enough to propel his side to victory and they must now overturn a 15-point deficit in the return clash next weekend.
In an open encounter Goneva opening the scoing in the third minute for Fiji but Tonga hit back with a well-worked try of their own through fullback Viliami Iongi. Nasiga scored Fiji's second try which Bai converted.
Murimurivalu carved his way through the Tongan defence to give Fiji a 21-5 lead at the break. Longi claimed his second try just a minute after the restart but two penalties from Bai made the game safe for Fiji.