Raducanu has struggled for fitness since her stunning US Open triumph in 2021 but looked at the top of her game as she pummelled Mertens into submission from the baseline, shouting and fist-pumping almost every point won.
The British wildcard raced into a 5-0 lead against her opponent under the closed roof of Court One which was packed with a passionate home crowd.
Mertens did get on the scoreboard to avoid a bagel, but that was the only positive for her as Raducanu clinched the first set.
The Briton then broke in the third game of the second set with a brilliant backhand across court and broke again to go 4-1 up.
Mertens, ranked 33 in the world, held to love in the next game but could not stop Raducanu from bringing up two match points with an ace and then winning the first of them after the Belgian went long.
It's the first time Raducanu has reached the third round of a Grand Slam since her title win at Flushing Meadows and raises tentative hopes of a first British women's singles triumph at Wimbledon since Virginia Wade in 1977.
"I think I’m playing really good tennis. I’m really happy with the improvements I’ve made," Raducanu said on court. "I knew all the hard yards and hard work I was doing this year would lead to something. I’m just so happy I’m able to reap some of the rewards here at Wimbledon."
