Carlos Alcaraz said his failure to capitalise on Novak Djokovic's thigh problem was his "biggest mistake" in losing to the 24-time Grand Slam champion in an Australian Open blockbuster on Tuesday.
The 10-time Melbourne Park winner Djokovic outlasted the younger Spaniard 4-6, 6-4, 6-3, 6-4 to set up a semifinal with Alexander Zverev.
The 21-year-old Spaniard Alcaraz has won four major titles but his wait for a first Australian Open crown goes on.
A wincing Djokovic, 37, called the trainer and had a medical timeout late in a first set which he went on to lose.
He returned with heavy strapping on his left thigh, but after moving gingerly at first, Djokovic only seemed to get better from then.
Djokovic afterwards refused to say exactly what his injury problem was, but said "the medication started to kick in, and it helped".
Alcaraz felt he should have rammed home the advantage when Djokovic was struggling still in the second set.
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"Honestly I felt like I was controlling the match and I let him get into the match again. I'm going to say that was the biggest mistake that I made today," said Alcaraz.
"In the second set I had to play a little bit better just to push him even more to the limit. We saw that he had issues just moving a little bit in the second set.
"I had to push him a little bit more just to the limit, and I didn't. I didn't do it.
"After that, I think he started to feel better and was playing such a great level."
Alcaraz said he would leave Australia with his "head up", saying that the defeat in a quarterfinal at the hands of one of the best ever would help him develop further as a player.
"I'm just lucky to live this experience," he said.
"From these matches I'm getting so much experience about how to deal with everything. I'm not going to hide.
"I've done great things in tennis already, but playing against one of the best in history of our sport, these kind of matches help me a lot in the future to be better."