John Isner comes to the net for some great shots and wins the tiebreaker to force a fourth set against Frances Tiafoe in the first round at the US Open. (0:22)

NEW YORK -- A week ago, John Isner saw his three-year run as the United States' No. 1-ranked man come to an end. On Monday, the same day that top ranking was restored, the No. 20 seed at the US Open was stretched to his outer limits by an 18-year-old fellow American.

Frances Tiafoe, the youngest player in the men's draw, had several choice looks at this first-round match, but Isner came back from a two-set deficit to beat his good friend 3-6, 4-6, 7-6 (5), 6-2, 7-6 (3).

On a torrid day when the temperature soared into the 90s at the National Tennis Center, the two players were on court for a draining 3 hours, 27 minutes.

Behind 35 aces, John Isner came back from two sets down to defeat Frances Tiafoe. Photo by Alex Goodlett/Getty Images

In a time of transition in men's tennis -- Roger Federer is notably absent here for the first time this century -- Tiafoe's near victory underlined the incoming blast of fresh air from the next generation. Tiafoe is ranked No. 125 among ATP World Tour players, some 104 spots below Isner, and is the 13th-ranked U.S. man.

Before the tournament, Tiafoe danced to hip-hop music with Gael Monfils on stage at a food-tasting event and declared he was happy to be playing the 31-year-old Isner, who was riding a 10-match winning streak in the first round of Grand Slams.

Even this stage, the inaugural men's match on the spanking-new 8,200-seat Grandstand Stadium, was not too big for Tiafoe, his lime-green ensemble and fearless forehand. After scorching a cross-court winner to force the third-set tiebreaker, the teenager broke into a smile and playfully gestured to the raucous crowd with both hands for more applause.

Believe it or not, Tiafoe outpointed Isner 149-148.

"One guy was 13 years older than his opponent, but it was an absolute pleasure," Isner said, after embracing Tiafoe and telling him to keep it going. "His backhand is world-class. His backhand return is world-class. He was handling my serve better than anyone, really, maybe outside of Novak [Djokovic]. I mean, he was really on it."

Previously, Tiafoe had won only a single ATP-level match this year (over fellow American teenager Taylor Fritz at Indian Wells), but he created some late-summer momentum by reaching the final in the Lexington, Kentucky, Challenger, losing to American Ernesto Escobedo. Tiafoe won his first Challenger title a week later in Granby, Canada.

Three weeks later, Escobedo, 20, and Tiafoe provided first-round highlights in their home Grand Slam. After splitting four sets with Lukas Lacko, Escobedo won by retirement Monday.

In another All-American battle, No. 26 seed Jack Sock outlasted the up-and-coming 18-year-old Fritz, 7-6 (3), 7-5, 3-6, 1-6, 6-4. The match ran a robust 3 hours, 21 minutes.

Tiafoe's poise was remarkable. The 6-foot-10 Isner is the ATP World Tour's leader in service rating, and he stroked 35 aces. Still, Tiafoe managed to create 11 break-point opportunities -- and he converted four of them. On the other side, Tiafoe actually won one more service games and, according to tournament statistics, stroked one offering 142 mph -- 3 mph faster than Isner's fastest.

"It hurts," Tiafoe acknowledged after the first official five-set match of his young life. "I can't wait to come back here next year."

Tiafoe grew up around the game as the son of a custodian for a tennis academy in College Park, Maryland. He won the USTA Boys' 18s national championship last year and received a wild card into the US Open, where he lost in the first round. He's played mostly Challengers this year but received another wild card into this year's tournament.

Experience was the difference for Isner, who is closing in on his seventh straight season in the top 20.

"Just gotta stay with it," Isner said of his comeback. "You know, I've done a lot of good things in my career. [I] tried to enjoy the atmosphere. Sort of tipped the match in my favor.

"I was obviously a bit tired, but that gave me a lot of energy going forward."

Isner, who has been to one major quarterfinal -- here in 2011 -- tends to play long matches in the early going, which wears him down in the third and fourth rounds. It happened again Monday, but it turns out his second-round opponent on Wednesday, Belgium's Steve Darcis, needed 4 hours, 11 minutes to dispatch Jordan Thompson.

After grinding it out against Tiafoe, Isner couldn't say enough about the teenager.

"I think the best thing he has going for him is he's just an incredible athlete," Isner said. "You can't really teach that. He's got wheels. He's got the hands. He's got shots on both sides.

"One area, if he improves his second serve a little bit, I would certainly buy stock in him right now, for sure. He has a fantastic future."