The Spaniard emerged victorious 7-6 (10/8), 6-3, 4-6, 6-4 after over three hours of blistering serves, power-hitting and drop-shots on Court Philippe Chatrier.

"I think we both have huge respect to each other, we entertained the people well," Alcaraz said on court.

"He's a really powerful player -- he can make any shot... I appreciate the moment that I've shared with him today."

Another American awaits Alcaraz in the last eight after 12th seed Tommy Paul earlier defeated Alexei Popyrin in straight sets.

The four-time Grand Slam winner edged a tight first set, which produced only one unconverted break point, at the end of a lengthy tie-break that saw both players have chances to clinch the opener.

When Alcaraz finally forced a break against the big-serving Shelton in the second set, it was enough for him to move into a two-set lead.

But once again Alcaraz failed to see out a straight-sets victory as Shelton produced two breaks of serve to edge the most open of the four frames.

The 22-year-old nonetheless bounced back to gain the advantage in the fourth set, before serving out as dusk descended over centre court.