"It was an amazing evening, an amazing result," said ten Hag. "In the beginning we very strong, we were completely dominating the game and we made a fantastic goal."
Tottenham boss Mauricio Pochettino admitted he was at fault for the Spurs line-up. "I can accept it was a mistake the shape we used," said the Argentine. "But there were not too many options."
Spurs, who were playing in their first semi-final in the Champions League, were without injured star striker Harry Kane. The South Korean forward Son Heung-min was also absent through suspension.
"I am not happy," added Pochettino. "But our lack of energy from the beginning made our game difficult. We were a little sloppy."
Ajax advanced to the semi-final on the back of away wins at Real Madrid and Juventus. They showed impressive sang froid in those triumphs and the same calmness was on display in the prelude to their goal.
Hakim Ziyech's through ball picked out van de Beek who took his time to leave Spurs goalkeeper Hugo Lloris prone before slotting into the bottom corner.
"The way we conceded the goal was so painful," said Pochettino. "I think we were so poor in this action. They showed more energy than us."