The 21-year-old Belgian wildcard, ranked 72nd in the world, pushed the world number six deep into a second-set tie-break, but Shelton dug in to hold him at bay and secure victory in just over one hour and 45 minutes.

Shelton, who blasted four aces and saved three of three break points across the match, will face Brazilian rising star Joao Fonseca in the last eight.

The American is still chasing a first clay court title, having lost the final in Munich last year to Alexander Zverev.

Three-time champion Zverev, who is again the top seed on home soil, faces Canadian Gabriel Diallo in their last-16 clash.

Earlier on Wednesday, French world number 26 and seventh seed Arthur Rinderknech was eliminated by Fonseca 6-3, 6-2.

The 19-year-old Brazilian also beat Rinderknech in Monte Carlo earlier in April.

Fourth seed Flavio Cobolli saw off Zizou Bergs 6-2, 6-3 to advance to the quarter-finals where he will face 78th-ranked Vit Kopriva after the Czech won 2-6, 6-4, 6-1 against the sixth seed, Luciano Darderi.