Denis Shapovalov said he was "incredibly ashamed and embarrassed" after his default for hitting umpire Arnaud Gabas in the eye with a ball handed Great Britain Davis Cup victory over Canada.
The 17-year-old was trailing Kyle Edmund 6-3 6-4 2-1 and had just been broken in the third set when he smashed a ball in anger that struck Gabas.
It was clearly not intentional from the Wimbledon junior champion but, with Gabas in obvious pain, tie referee Brian Earley had no choice but to halt the match and leave Britain the victors.
There were boos from the crowd at the TD Place Arena, who had earlier roared Vasek Pospisil to a 7-6 (7/3) 6-4 3-6 7-6 (7/5) victory over Dan Evans that set up the deciding rubber.
An emotional Shapovalov, who was playing in only his second Davis Cup tie, said: "I went back and spoke to the umpire afterwards and apologised directly to him.
"Luckily he was OK but obviously it's unacceptable behaviour from me.
"I feel incredibly ashamed and embarrassed and I just feel awful for letting my team down, for letting my country down, for acting in a way that I would never want to act.
"I can promise that's the last time I will do anything like that. I'm going to learn from this and try to move past it."
The International Tennis Federation announced Gabas had bruising and swelling to his left eye and had been sent to Ottawa General Hospital for a precautionary evaluation.
The incident brought back memories of the 2012 final at Queen's Club, when David Nalbandian was defaulted for kicking an advertising hoarding against the leg of a line judge.
Tim Henman, meanwhile, was defaulted at Wimbledon in 1995 after striking a ball at a ball girl.
Canada captain Martin Laurendeau accepted that the decision was the only possible course of action.
He said: "I didn't see what happened. When the last point was over I got up to see if on the changeover he'd have all his drinks and bananas and I just heard the crowd go quiet and then I looked back to see what had happened to the umpire and that's when I realised he was in the middle of it all.
"I knew immediately the rules are the rules and you've got to play by the rules."
Laurendeau, though, said he would have no qualms about picking Shapovalov for future ties and backed him to learn from the experience.
"He's a kid, he wants to face the music, he's not going to shy away," said Laurendeau.
"He's got some great talent and it's just the beginning of his career. He'll draw a big lesson out of this.
"Hopefully this makes Denis a stronger person, a better player but a better person, and he's already a great kid."