Frances Tiafoe after defeat to unseeded Ben Shelton

 In what was an all-American US Open quarterfinal, Frances Tiafoe suffered a heartbreaking defeat to fellow countryman Ben Shelton but, in a gesture acknowledged by a full house at the Arthur Ashe Stadium, the 25-year-old lauded the opponent who took him down on the night.

Shelton became the youngest American US Open semifinalist in 30 years after coming through 6-2, 3-6, 7-6(7), 6-2 against his more fancied compatriot Tiafoe.

In a courtside interview by the host broadcaster, Tiafoe said the fact that Shelton went into the last-four clash as an underdog actually worked in his favour on the night.

Significantly, Tiafoe, currently ranked 10th by the ATP, was the underdog when he took down Argentine Diego Schwartzman, 14-time French Open champion Rafael Nadal, and Russian Andrey Rublev at the US Open last year.

However, going into the semifinal clash as the 10th seed, the burly American was expected to prevail over his unseeded fellow countryman.

"It's a different seat to be in. Obviously Ben really wanted to win. Ben came out and played with a lot of energy. But it was different conditions today. It was super heavy. Not what I was accustomed to when I played at night earlier in the week," Tiafoe was quoted as saying in the official portal of the US Open.

"It's definitely different. Obviously, a lot of times I play late in tournaments, I've been the underdog, so I just go out and play. Kind of like how Ben did, play and swing and do whatever you want," he added.

"But even so, I still put myself in positions and I just didn't play well enough," the American said.

Once they entered the court, Tiafoe claimed he was able to put the occasion and his friendship with Shelton—who called him his "elder brother"—"to the side".

"Once you get out there, you're just going to play to win, obviously. Obviously, it's a big match. You're playing to get in the semifinals," he said.

"This game is a very selfish game. Once you get out there, you just want to win and try and bring your best stuff to win a match, and then your relationship stays after the match. I don't think it really mattered. We both really tried to come after each other and he just played better than I did today," the American added.