I just wasn't myself, says Djokovic after stunning loss in US Open

World number one Novak Djokovic had no explanation for his flat performance in a stunning semi-final loss to Japan's Kei Nishikori at the US Open on Saturday.

"I think he just played better in these conditions than I did," the Serb said of the 6-4, 1-6, 7-6(4), 6-3 setback in brutal heat and humidity on Arthur Ashe Stadium.

"I just wasn't managing to go through the ball in the court," Djokovic told reporters.

"I wasn't in balance. Unforced errors. Even when the ball gets back to his part of the court it's pretty short; he takes advantage of it. On the other side I didn't. That's it.

"Other than that second set, my game today was not even close to what I wanted it to be. A lot of unforced errors, a lot of short balls. Just wasn't myself."

Straining to make sense of his showing, reporters asked seven-times grand slam champion Djokovic whether he has been distracted by the approaching birth of his first child.

"Of course, I'm in touch with my wife. Of course, I am being part of her pregnancy. It's normal," he said. "But it's no different for me in terms of preparing for the match and for the tournament.

"I had all my dedication, everything directed to this US Open. I tried to give my best and I did. Now my best today was not where I wanted it to be."

The Wimbledon champion gave full credit to the 10th-seeded Nishikori, who became the first male Asian player to reach a grand slam singles final.

"He played some great tennis. I congratulate him for the effort," the Serb said. "He was the better player today."

Djokovic said it was a great day for Japanese tennis.

"This is definitely huge for Japan," he said. "This can definitely be a great encouragement for tennis in that country.

"He's been around for last couple of years. He's been making a lot of success. But playing finals of a grand slam and now fighting for the title is definitely something different. He has gotten to another level."

As for his future plans, Djokovic sounded unsure.

Asked about whether he would play in Serbia's Davis Cup matches next week against India, he said: "I have to speak with my captain and see, see if I'm going to go in India or not. I still don't know."

He also said was it was too early to tell whether his US Open exit could be the last time he plays this year.

"I don't know yet," Djokovic said. "Just got off the court. I don't know."