Defending US Open champion Carlos Alcaraz continues to be impressed by Novak Djokovic.

Alcaraz booked his place in the last 16 of the tournament following a 6-2, 6-3, 4-6, 6-3 victory over Britain's Dan Evans on Saturday.

Djokovic had a much tougher night on Friday as he came back from two sets down against compatriot Laslo Djere to win 4-6, 4-6, 6-1, 6-1, 6-3 in a contest that lasted nearly four hours.

Live and breathe sports?

🚨 Get viral graphics, memes, rumors and trending sports news delivered right to your inbox with the Clutch Newsletter.

Of course, when any player goes down two sets, there's a high probability of them going on to lose the match. However, Djokovic is not like most players and Alcaraz was not surprised to see him come back and win.

“I watched the first two sets, then I had to go to go to sleep,” Alcaraz said (via Tennis 365). “When I went to bed I was thinking that Novak is going to come back, that’s for sure. He shows once again that he’s one of the best in history. I think I saw [stats that he has done it] like six, seven times. It’s something crazy.

“Being 36 years old and doing the same things as when he was 20 is incredible. And we have to give credit for that and it’s something admirable.”

Djokovic will play Croatia's Borna Gojo next while Alcaraz plays Matteo Arnaldi for a place in the last eight. Arnaldi reached this stage after the 22-year upset 16th seed Cameron Norrie with a 6-3, 6-4, 6-3 win.

The Spaniard admits he's not seen much of Arnaldi, but will do as much research as he can prior to their match.

“The truth is that I haven’t been able to see him much,” Alcaraz added. “Although it is true that I had the opportunity to see him when he played the Challenger in Murcia, because it was in my country. It’s been a while and I haven’t been able to see him much more. If he is in the fourth round beating players like Norrie, he deserves to be there.

“He is a very, very tough player. We’ll see how he plays on the rest day. I’ll try to watch some of his videos, some of his games that he has played and, from there, see how we are going to play against him. Obviously I know that it is going to be a very, very tough game, in which I have to demand a lot of myself.”