Novak Djokovic struggled a little against the serve-and-volley game of Feliciano Lopez, and there was a time, during the second set, where it looked like we might have another major US Open upset. But the world number one showed that mental strength and unerring ability to play the big points well, yet again, to move into the semifinals at Flushing Meadows.

Djokovic was made to work hard for his win against Roberto Bautista Agut in the previous round, and this one was no different as the Spaniard Lopez played some outstanding tennis; just not enough to beat the near unbeatable Djokovic.

The world number one eventually prevailed 6-1, 3-6, 6-3, 7-6 (7-2).

Djokovic started the match like a freight train, going up two breaks in a hurry and it looked like everyone would get a relatively early night's sleep.

However, that blink-of-an-eye tennis momentum-swing took hold again, with Lopez roaring back in the second set, leaving Djokovic befuddled in that period of play.

But then, was there ever any doubt that Djokovic would come back?

The master counter-puncher found his groove again, and even if Lopez kept playing some good tennis, Djokovic took the third set to take a crucial lead in the match.

The final set went back and forth, with both players hitting some outstanding shots. There were a couple of moments in the match where the players were applauding each other for some outrageous tennis.

First Djokovic got a little clap on after a brilliant serve, volley and putaway by Lopez, before, soon after, all Lopez could say was "wow" as Djokovic gave his opponent a taste of his own medicine with a delicious drop volley.

With neither giving an inch on serve in the final set, especially the second half, the tie-break was inevitable, and you always knew Djokovic would be the one to hold his nerve better. So it proved to be as Djokovic got a couple of mini-breaks from 2-1 to move to 4-1, before easily closing out the set and the match as Lopez hit a backhand into the net.

Waiting in the semifinal now is the defending champion Marin Cilic, who won a marathon five-setter against Jo-Wilfried Tsonga 6-4, 6-4, 3-6, 6-7 (3-7), 6-4.

"I came to the tournament knowing that I can play well here, that I just need few matches to get into the rhythm, and that's what happened," Cilic said. "I was feeling that I was starting to hit the ball much better.

"In the previous match with [Jeremy] Chardy, I finished with the third and fourth set really strongly. I was playing really good tennis. Today, to beat Jo with a demanding day like this, it's of course a huge accomplishment."