Mauricio Pochettino says his players took their foot off the gas at 3-0 in their 3-1 victory over Southampton on Wednesday, allowing their visitors a late flourish at Wembley.
Harry Kane, Lucas Moura and Son Heung-min all scored inside 55 minutes to put Spurs firmly in control, with new Saints boss Ralph Hasenhuttl watching from the stands.
But the visitors were not without their own chances and hit the woodwork three times, as well as drawing a superb double save from Hugo Lloris before Charlie Austin netted a late consolation.
"Tough game, it is true," he said after the match. "I was saying that maybe we conceded more chances in the last 30 minutes than in 90 minutes against Arsenal.
"Yes, with 3-0 maybe we drop a little bit in our concentration. I think we start to think the game was over and always it is so dangerous when there is so much time to play.
"Of course in the end, Lloris was man of the match because it was an unbelievable action that save, but that we cannot afford.
"In the same way I try to understand, it was a tough evening, difficult to play the three emotional games we played last week. You pay. It is not easy to feel fresh in your legs or your mind - play or not play, players that don’t play are the same.
"For me the most important thing is to keep going, 33 points, third in the table and I think a fantastic Premier League season."
The Spurs boss also gave an update on Kieran Trippier, who left the match with an apparent knock late on.
"I think it's the same problem, the same problem that started in the World Cup," he said of the 28-year-old. "If you remember the semi-final against Croatia, when he left the pitch. Then I think to play after three days against Belgium, I think maybe it affected a little bit.
"Now he's suffering in that situation. In situations like today when the pitch is not in a good condition, this area, the groin, suffers more when the pitch is different.
"It's a thing that we need to care a lot for him, protect him, he needs to work a lot to protect and sort that problem. It will be a long period for him suffering this type of problem.
"When I was in Argentina as a 19-year-old I suffered and it is an area that is difficult to sort out. But I don't believe it's a big thing. It's only he needs to start to accept that situation and live with sometimes some pain and play."