London -- Tottenham manager Mauricio Pochettino was uncharacteristically critical of his side's mentality after throwing away a lead to lose 2-1 at Southampton on Saturday and plunge their chances of Champions League qualification into further peril.
Spurs have now taken just one point from their last four Premier League games to see their lead over Manchester United, Arsenal and Chelsea in the fight for a top four finish diminish.
Pochettino was serving the first of a two-match touchline ban on his old stomping ground and was particularly frustrated that his side failed to build on their first-half dominance after Harry Kane's 200th career goal opened the scoring.
"It is so difficult to understand why we changed so much from the first half to the second. I am very disappointed, we were better in everything and then we lose the game," said the Argentine.
"It is about competing better for the first goal if you want to be a contender for big things.
"The second-half we did not take it seriously. I don't want to criticise too much but when you lose a game that was under control after 45 minutes, should be 3-0, 4-0 and finishes in 2-1 defeat, it is difficult to accept.
"We started to take some risks that were unnecessary and that was the main problem. It gave them belief and they started to build their game.
"When you lose the momentum after 45 minutes, that is about mentality, that is about being strong and keeping the right. For us, we lost the momentum and gave them relief."
Goals from Yan Valery and James Ward-Prowse in the final 14 minutes gave Southampton a vital three points in their bid to avoid relegation.
The sloppy performance came on the back of the midweek victory at Borussia Dortmundthat secured Tottenham's place in the quarter-finals of the Champions League.
And Pochettino suggested his players were punished for resting on their laurels.
"It's about complacency. It's about arrogance in a bad way," he said.
"You use the first half like an example, playing with intention, focus, concentration, we matched them in everything and we were much better.
"In the second half it was arrogance in a bad way. I think it is a very good example for everyone of how you need to play for 45 minutes and how not to play for 45 minutes.
"That is of course the responsibility of everyone. I put myself ahead of everyone. I don't want to blame someone or the team. I want to blame all of us, the club of course."
Spurs now lead Manchester United by three points with Arsenal a point further back and Chelsea five points behind Tottenham.
However, Chelsea have two games in hand, at home to Brighton and Wolves, while United and Arsenal go head-to-head in their game in hand on Sunday.
"People talk about perceptions but we need to realise if you do not fight with intensity for the Champions League, you will struggle," added Pochettino.
"We are fighting against big sides (for the top four). We had a big gap, welcome to the reality, we are under pressure to win games. It is a massive challenge for us."
Southampton manager Ralph Hasenhuettl believes it was vital his side won to remain out of the bottom three before a period when they have no fixtures due to the FA Cup and the international break.
"It's better to go into the three weeks break with a win that keeps us over the relegation line. The last break we were below the line and that was horrible," he said.
"We lost against Man United 3-2 after we played a sensational game. We only earned warm words and I said today we need to earn more than warm words.
"It was a moment to celebrate at the end with the supporters and players but also to tell them it was only one step. a big one perhaps. But we still have a long way to go."