NEW YORK, Sept 6 (Reuters) - Unseeded South African Lloyd Harris stormed into his first Grand Slam quarter-final with a crushing 6-7(6) 6-4 6-1 6-3 victory over American Reilly Opelka on Monday. The 24-year-old Harris, whose previous best performance at a major was his run to the third round of the Australian Open earlier this year, will next face fourth seed Alexander Zverev for a place in the semi-finals. Harris, who dispatched seventh seed Denis Shapovalov in straight sets in the previous round, won an incredible 92% of his first-serve points, smashing 36 aces and 62 winners to seize the initiative against Opelka. "It was a really tough match," Harris said. "Reilly is always going to come with a lot of big serves, that"s for sure. "I think I handled it well... I held my composure. I served really well throughout the match. All in all, I"m just very, very pleased and very relieved after that performance." Harris had the best of the early exchanges, but neither player was able to gain a clear advantage until the South African pounced midway through the opening set to break Opelka"s booming serve and take a 5-4 lead. Opelka, the last American left in the draw, broke straight back and held his nerve in the ensuing tiebreak to go 1-0 up in the match, but that was as good as it got for the 22nd seed. The second set stayed on serve until the very last game, with Harris getting the crucial break to level the match, before turning up the heat and pulling away from his opponent in the final two sets. He broke Opelka six times in the match, while only losing his own serve once, keeping things tidy with just 16 unforced errors to close out the best result of his career. The 6-foot-11-inch Opelka packs one of the most powerful serves on the men"s tour, but struggled to bring his most potent weapon to bear on the contest, landing only 65% of his first serves, though he still managed 24 aces. "I don"t think I served my best. I actually served poorly, to be honest," Opelka said. "He served great. He was the better server. "It was an uncharacteristic serving day for me. That changed the whole thing. I mean, even if it was a characteristic serving day, I wasn"t going to break him much. He was serving unbelievable."