Mauricio Pochettino admitted Tottenham blew their big chance to keep the title race alive after the 1-0 defeat at West Ham.
Spurs travelled to the London Stadium looking for a 10th straight victory, but instead Manuel Lanzini's strike all but handed the Premier League crown to Chelsea.
The Blues will now be seven points clear of Spurs if they beat Middlesbrough at home on Monday - and could even wrap up the title against West Brom next Friday before Tottenham kick a ball again.
And boss Pochettino conceded his side had let their opportunity slip as, for the second season running, they look set to come up short.
"You need to show in the moments to win a league," he said. "Today was a key moment, we had to show that when you do not play well but when you must win, you win.
"If you want to be a champion you have to win. How? I don't know. But you have to win.
"We try to move on and finish the season in the best way, still fighting, but now it's true, it's more difficult.
"I feel calm. I'm disappointed, of course, that we missed the opportunity to reduce the gap to Chelsea."
West Ham have endured a torrid season but it seems there is nothing like a home game against Tottenham to rouse them and their fans - just a year ago a 1-0 defeat at Upton Park put a major dent in Spurs' eventually unsuccessful title bid.
They nullified the threat of Christian Eriksen, Dele Alli and Harry Kane with the latter's only real chance forcing a fine double save from home goalkeeper Adrian.
"It wasn't our best day," added Pochettino. "The performance was just okay. Maybe we didn't deserve to lose, but we did because we conceded a goal.
"It's difficult to be focused on Chelsea now. If you're not able to win and put pressure on them then whatever happens is the worst.
"Now it's better to forget a little bit and see what happens. We have to try to analyse and understand why it was difficult for us to get the three points here today."
This could also be the game where West Ham finally started to feel at home in their new stadium.
For all the talk of its lack of atmosphere, as Lanzini smashed home the second-half winner a roar went up like nothing heard at this venue since Mo Farah came down the final straight in 2012.
The victory means the Hammers are now guaranteed to stay up and eased the pressure on manager Slaven Bilic, who is still dealing with constant speculation surrounding his future.
"We deserved to win," said Bilic.
"We had a game plan, but the way we did it was magnificent. A great team display in terms of character and determination.
"To beat a team like Spurs you need more than that and we also showed quality.
"It was an important one for them and us, and under the lights on a Friday night, against them - you can't beat that feeling."