Meanwhile, Shelton remained on a championship collision course with Zverev. A day after erasing three match points to topple Borna Gojo, the second seed took a more direct approach to the finish line.

Shelton defeated two-time runner-up Botic van de Zandschulp, 7-6 (1), 6-3, claiming 83 percent of his first-serve points.

“I’ve figured out how to manage my serve, raise my first-serve percentage a bit and be winning a lot off of my first serve, I’m happy,” assessed Shelton with ATP Media after noting the higher altitude this event presents. “There’s a lot that I can continue to build on going forward in this tournament.”

The left-hander improved to 12-7 on the year in reaching his third quarterfinal. Luciano Darderi, who rallied past Miomir Kecmanovic with a 2-6, 7-6 (4), 6-4 effort, awaits.