WIMBLEDON, England — Reilly Opelka still is getting used to some aspects of visiting Britain.

The bed frame where he stayed during a Wimbledon tuneup tournament that wasn’t quite big enough for the 6-foot-11 American, for example. The lack of air conditioning compared to back home in Florida. As for the grass-court tennis? Opelka is increasingly comfortable with that, as demonstrated by a 7-5, 3-6, 4-6, 6-4, 8-6 second-round victory over three-time major champion Stan Wawrinka at the All England Club on Wednesday.

“Every year I come here, and, like, ‘Why is there no AC?’ They’re, like, ‘Oh, it’s never hot here.’ Every year, it’s over 80 degrees!” Opelka said.

He is ranked 63rd and is unseeded for his main-draw debut at Wimbledon, where he won the 2015 junior title.

But against a far-more experienced and far-more accomplished player, he pounded eight of his 23 aces and saved both break points he faced in the last set, fending off 10 of Wawrinka’s 12 break chances in all. At 6-6 in the fifth, Opelka erased a break chance with a 140 mph service winner, then followed with two more serves Wawrinka failed to put in play, at 133 mph and 137 mph.

It was Opelka’s ability to adjust his style, serve-and-volleying only three times in the final set, and to hang in there on longer exchanges that made this upset possible.

“I’ve always been a pretty good mover. Underrated, actually,” said the 21-year-old Opelka, participating in only his fourth major tournament.