Canada's Denis Shapovalov said he was "incredibly ashamed and embarrassed" after he was defaulted for hitting the umpire with a ball - handing Great Britain Davis Cup victory in Ottawa.
The 17-year-old had just dropped serve to trail Kyle Edmund 6-3 6-4 2-1 when he angrily hit the ball out of court.
It struck French umpire Arnaud Gabas in the eye and a default followed.
"Luckily he was OK but obviously it's unacceptable behaviour from me," said Wimbledon junior champion Shapovalov.
"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 World Group first-round tie was poised at 2-2 after Vasek Pospisil beat Dan Evans to set up a decider, but Canada's hopes ended when Shapovalov let frustration get the better of him.
He later apologised to Gabas in the referee's office before the Frenchman headed to Ottawa General Hospital for a precautionary evaluation on bruising and swelling to his left eye.
The International Tennis Federation (ITF) said in a statement it was "clear that Mr Shapovalov did not intend to hit Mr Gabas".
Referee Brian Earley has the power to impose a fine of up to $12,000 (£9,600) and the ITF might significantly increase that fine, and suspend Shapovalov from future ties.
GB captain Leon Smith said: "Unfortunately for the young lad this is going to get an awful lot of attention.
"This will be looked at closely and it should be as it is dangerous. Whether it's an umpire or a young kid who's at the side of the court, that really could be a serious injury, so I'm sure it will be dealt with swiftly and pretty firmly."
Britain go on to face an away tie in France from 7-9 April - a repeat of the 2015 quarter-final in London that Britain won on their way to regaining the title for the first time in 79 years.
John Lloyd, former Great Britain Davis Cup captain and player
We don't know yet how the umpire's eye is but we could see it was already closing. You don't know about permanent damage until he sees the doctor.
It's devastating for Shapovalov. He let himself down, he let his country down. He could have caused serious damage to the umpire. He will realise that he can't do that sort of thing again and he's going to get a lot of trouble in the press for this, quite rightly so because he deserves it, but he will rebound.
If you look to the brighter side we've seen some undoubted talent in him, if he can just control it a little bit. There's nothing wrong with getting emotional - we've seen great champions like John McEnroe get emotional - but you can't go to that extent and he'll have to curb it a little bit.