London - Mauricio Pochettino took his share of the blame for "sloppy" Tottenham's 1-0 Champions League semi-final first leg defeat against Ajax after admitting he used the wrong tactics.

Pochettino's side made a dismal start as Donny van de Beek put Ajax ahead after just 15 minutes at the Tottenham Hotspur Stadium on Tuesday.

Ajax dominated the first half until Pochettino completely changed his game-plan by switching from a 3-5-2 formation to a 4-4-2 system.

Aided by the introduction of Moussa Sissoko for the injured Jan Vertonghen, Tottenham regained a measure of control without ever hurting Ajax with their long-ball barrage.

Pochettino conceded he had blundered with his game-plan, but the Tottenham manager was also angry with the lethargic way his players started the club's first European Cup semi-final for 57 years.

"Watching now of course I can accept it was a mistake the shape we used. But there were not too many options," Pochettino told BT Sport.

"I am not happy. But our lack of energy from the beginning made our game difficult. We were a little sloppy.

"It was not the shape that conceded the goal. Our approach to the game was not good. I am the manager so I have responsibility."

Tottenham were hamstrung by the absence of injured striker Harry Kane and suspended forward Son Heung-min.

But Sissoko's dynamic contribution and the return of Son for next week's second leg in Amsterdam give Pochettino hope they can still reach the final.

"After we conceded the goal, 25 to 30 minutes in, we started to be in the game. Moussa Sissoko provided good energy," he said.

"Second half we pushed them and tried to create chances. It was an even game in the second half.

"We are alive. It's only 1-0 down. We need to believe we can go there and win the game."

It remains to be seen whether Vertonghen will be fit for the return after the centre-back appeared to suffer a concussion in a clash with team-mate Toby Alderweireld.

Vertonghen went off for treatment, but was then surprisingly sent back onto the pitch by Tottenham's medical staff.

He looked groggy and on the verge of throwing up before being helped down the tunnel after signalling he couldn't continue.

Asked about the severity of Vertonghen's problem, Pochettino only said: "I didn't see him. Of course, we will assess him in the next few days."