Ajax head coach John Heitinga rued more frustrations in a disappointing season after Feyenoord came from behind to strike a significant Eredivisie title blow.
Arne Slot's side moved six points clear at the Dutch top-flight summit after Lutsharel Geertruida scored an 86th-minute winner in a 3-2 victory over second-placed Ajax on Sunday.
It marked the first time since August 2014 that Ajax lost an Eredivisie game they had led at half-time, after Edson Alvarez and Dusan Tadic goals had sent the hosts in leading 2-1 at the interval.
With only the top two qualifying for the Champions League, Heitinga – in charge until the end of the season following Alfred Schreuder's sacking in January – acknowledged Ajax are struggling with AZ and PSV in hot pursuit.
"Let's be clear: this is not the season Ajax wants it to be," said former defender Heitinga, as quoted by De Telegraaf.
"I have been asked to help this club. When I started we were fifth and then we won seven league games in a row.
"And we probably would have won here too if [Mohammed] Kudus scored in the final phase. But unfortunately, he didn't. We are really going to do everything we can to finish this season as high as possible."
Stats Perform's predictor, which calculates the probability of a team's league finish based on numerous data, suggests Feyenoord have a 92.6 per cent chance of winning the league after a late win over Ajax.
Those title hopes were boosted by Geertruida's header, which was Feyenoord's latest winner in De Klassieker since November 1969 when Theo van Duivenbode scored in the 87th minute.
Feyenoord coach Slot said his side can dare to dream of a first championship since 2017 and just a second this century.
"We are in very good shape," he said. "I dare say that we are competing for the championship. And that we are in a fantastic position too.
"We had a great victory here. We know how much effort it takes to win every time. There is nothing wrong with that; we like to do that.
"I saw a physical performance today that also appeals to the imagination. If we can keep delivering and no one gets injured, then we are in really good shape."
Slot's men were somewhat fortunate in the first half, though, as Quilindschy Hartman escaped a second caution in quick succession from referee Danny Makkelie.
The defender was promptly removed in the 25th minute for Marcos Lopez, with the game finely poised at 1-1 after Alvarez cancelled out Santiago Gimenez's early opener.
"He got away with it, the referee could have given yellow again," Slot told ESPN. "Although you also have to analyse that first yellow card. Three violations preceded that.
"Everyone understands that you don't enjoy playing football anymore if you have committed an offence that could have been your second yellow card.
"Then you can no longer play against Kudus, a man who is so incredibly good in terms of individual class."