Having been stagnant in Gareth Southgate’s system in the first half, England controlled the second as Jack Grealish shone on his competitive debut, though neither he nor Harry Kane could forge a way back for the Three Lions.
Eric Dier, who survived a collision with Kyle Walker in the warm-up to start, looked to play an ambitious ball out of the back which was swiftly picked off by former Tottenham teammate Jan Vertonghen.
Belgium turned the tables quickly, finding Romelu Lukaku before Southgate’s side had sorted themselves out at the back, Declan Rice was left chasing shadows when the ball was squared to Tielemans, who fired past Jordan Pickford with the help of a deflection off Tyrone Mings and the inside of the post.
England threatened shortly afterwards as captain Kane, earning his 50th cap, looked to have steered a header in from a corner, only for Lukaku to clear off the line.
Southgate’s side were seeing plenty of the ball and still looking to keep things tight, but saw that plan blown out of the water when – after Belgium were awarded a dubious free-kick following a Rice challenge on Kevin De Bruyne – Mertens whipped a delightful free-kick up, down and in to double the lead and make a first defeat on home soil since 2016 look far less likely.
The visitors were dealt a further blow when Chelsea’s Ben Chilwell – making his first appearance for England in a year – was forced off injured and replaced by Arsenal’s Bukayo Saka before the break.
Southgate stuck with the often criticised three at the back shape for the second half, but opted for a change of personnel with Harry Winks replacing Jordan Henderson.
As he had done through the first half, Grealish continued to draw fouls and cause problems for the Beglian back line as England increased their influence.
Kane and then Grealish both won free-kicks on the edge of the area, though both came to nothing, before the Tottenham striker fired at Thibaut Courtois after a fine flick from Mason Mount into his path.
England were dominating the second half but still could not find a goal to get back into the game.
Southgate sent on Dominic Calvert-Lewin and Jadon Sancho, though remained wedded to the system with Everton striker deployed on the right of a front three and the Dortmund winger at right wing-back.