Roberto Martinez’s men were dominant from start to finish in a deserved victory and, while Romelu Lukaku also caught the eye, Mertens was on another level, scoring twice and setting up another two in an eye-catching display in Brussels.
Things looked ominous for Estonia very early on as Belgium cruised into a comfortable 2-0 lead inside 16 minutes thanks to goals from Thomas Meunier and Mertens.
Hazard opened up a three-goal cushion with a fine Mertens-inspired effort, though Henri Anier’s goal just before the half-hour mark saw Belgium’s defence breached for the first time in qualifying.
There was never any danger of a comeback. though, and Yannick Carrasco netted Belgium’s fourth after 62 minutes to round off a brilliant team move, before Ragnar Klavan directed a Mertens cross into his own goal.
The magnificent Mertens added a glorious sixth with 22 minutes to go and Lukaku wrapped things up with a late double as Belgium eased to a maximum 12 points from four matches, leaving them two clear of Greece.
Belgium’s preparations were not helped by injuries to Mousa Dembele, Vincent Kompany and Christian Kabasele, but they looked completely unaffected and effectively ended Estonia’s hopes inside the first 25 minutes.
Meunier broke the deadlock just eight minutes in, making the most of hesitant goalkeeping from Mihkel Aksalu to head Carrasco’s cross in at the back post.
Lukaku almost doubled their lead shortly after when he struck the post, but Belgium did not have to wait much longer for the second – Mertens seeing his fierce 25-yard drive deflect off Klavan and beat Aksalu.
Mertens then turned provider as the hosts moved 3-0 ahead, with the Napoli star feeding Hazard in behind the defence and the captain producing an emphatic finish.