Isco has found game time hard to come by this season with reports suggesting he has a fractured relationship with manager Santiago Solari.
Asensio, a 73rd-minute substitute, was unmarked at the far post as he tapped in a cross from the right by Dani Cavajal with just three minutes of regulation time to go.
Ajax thought they had taken the lead 10 minutes later when Nicolás Tagliafico nodded home from close range, but after consultation with the Video Assistant Referee (VAR) the goal was ruled out for an offside.
Bayern Munich and Ajax have both won the European Cup three times in a row but only Madrid have managed more, when they lifted five on the bounce in the 1950s.
"We did everything and that shows the strength of this group". "Not only the goal, which of course was very important for us".
Ajax were not finished, though, and with 15 minutes left they sliced Real open again with Ziyech converting a low cross from David Neres. You have to know how to read games and we did that. "We trust what VAR or the referees say", he said.
"I'm particularly happy with his goal and his performance", Solari said afterwards in his press conference.
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Solari said he had not seen the images. It's a shame that our goal was disallowed.
"I hope there will be many more and every day I get up excited to represent the team as captain".
Vinicius chased a ball over the top down the left wing and cut inside before finding Benzema, who sent an unerring finish across Anona and high into the net.
Ajax forward Hakim Ziyech missed the next chance after a subtle touch by Donny van de Beek.
Perhaps nobody typifies Madrid's turnaround under Solari more than Vinicius Jr, who has brought freshness to the attack, with the Brazilian's willingness to run at rivals a key factor in Karim Benzema's current goal-scoring form.
Ajax played well despite their defeat and Madrid were made to work for the win.