Britain's Andy Murray says he has never experienced a defeat like his 6-0 6-1 hammering by Roger Federer at the ATP World Tour Finals.
Murray, 27, was beaten in only 56 minutes by the Swiss world number two, ending his participation in the season-ending event at London's O2 Arena.
Scot Murray told BBC Sport: "It has not really happened to me before like that where winning games has been an issue.
"I've had tough losses and heavy defeats but nothing quite like that."
He added: "It's very frustrating, when I'm trying my best. I've never been in that position before in my whole career.
"Maybe I need to make some changes to my game, but that's the one good thing - I have time now to do that."
Murray required a straight-sets win to qualify for the semi-finals but claimed only eight points in the first set and did not win a game until 6-0 5-0 down.
It was his heaviest defeat since he lost 6-1 6-0 to Novak Djokovic in Miami seven years ago.
Murray trails Federer 12-11 in their head-to-head and is 0-9 in 2014 against Djokovic, Federer and Rafael Nadal, the world's three best players.
Asked how far off their level he was, Murray said: "After tonight, that's quite clear - quite a long way from that level.
"But a lot can change in a matter of weeks and months in tennis. Roger is a good example of that."
After a difficult year in 2013, when he struggled with a back injury, Federer, 33, has been in impressive form this season, winning more matches - 71 - than any other player on the men's tour this year.
The 17-time Grand Slam champion's only defeat since the US Open in September was against Milos Raonic at the Paris Masters two weeks ago.
Federer's coach, Stefan Edberg, a six-time Grand Slam champion, said it was the best he had seen the Swiss play this year "by far".
"I don't think he can play much better to be honest - and I've seen a lot of tennis over the years," said Edberg.
"It's just one of those performances where everything goes your way, so it was enjoyable. He's in good form.
"Last year was not a great year for him, but he has been able to practise and work hard for a long period of time and win a lot of matches."