All this fuss about a roof at the U.S. Open, but the most significant consideration on the tournament’s first day may have been John Isner’s ceiling.

The top-ranked American male for most of the past five years, the 31-year-old Isner almost was shown the door by 18-year-old wild card Frances Tiafoe before prevailing in a fifth-set tiebreaker, 3-6, 4-6, 7-6 (5), 6-2, 7-6 (3).

Though he has been hanging around the world’s top 20, off and on, since 2010 and levitated as high as No. 9 of a few occasions, Isner has advanced as far as the quarterfinals — and then only once, at the 2011 Open — in 33 previous major-tournament appearances.

And the way Monday’s match played out echoed his entire career, raising the argument that, were it not for his searing serve, the 6-10 Isner probably couldn’t think about measuring the drapes to ever reside in the Grand Slam hall of champions. Or even take up lodging among the top 50 players.

Tiafoe’s spunky performance was not entirely shocking. The Maryland son of Sierra Leone parents, he took up the sport when he was 4 years old at the College Park junior tennis center where his father worked on the construction crew and later as custodian. He is the reigning national 18-and-under champ and one of four U.S. teenagers who were lurking in the men’s main draw as a possible future stars—Michael Mmoh Jared Donaldson and Taylor Fritz, who was beaten in five sets by another young Yank, 23-year-old Jack Sock.

“He’s got wheels,” Isner said of Tiafoe. “He’s got the hands. He’s got the shots on both sides . . . I certainly would buy stock in him right now, for sure.”

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In the end, when Isner embraced Tiafoe in a bear hug, “He told me, ‘You’re going to be great. Don’t let this get you down,’ ” Tiafoe said. “I was kind of crying on his shoulder.”

But the story was how Isner needed all of his 35 aces, the firewall against Tiafoe’s nimble court coverage and constantly shifting pace. In 90-plus heat and suffocating humidity, Tiafoe — never timid — somehow managed to freeze Isner repeatedly in place, dramatically varying his own serve, from a high of 142 miles per hour to one at 79 (a service winner), and whipping cross-court winners that left Isner flat-footed.

They were chasing each other around the new Grandstand Court, which seats in excess of 8,000 and was overflowing by the time they reached the final set. “Standing room only,” Isner noticed. “The crowd was going nuts. A lot of people were cheering for him. Rightfully so.”

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Tiafoe was ahead, 3-1, in the third-set tiebreaker but Isner’s serve saved him. From a 5-3 lead in the fourth, Tiafoe again had his upset soufflé collapsed by Isner’s serve. Serving at 5-6 to force the decisive fifth set tiebreaker, Isner hammered four consecutive aces and added another in the tiebreaker.

“I was struggling to match his intensity,” Isner said. “Just an incredible athlete. You can’t really teach that.”

As for himself, Isner will “stay the course.”

At least that serve isn’t going anywhere.