LONDON—After losing to Novak Djokovic in the semifinals of the ATP World Tour Finals, Kei Nishikori lauds the Serb, who clinched the year-end No. 1 ranking earlier in the week.

"When he's playing well, I don't think anybody can stop him," said Nishikori, who lost 6-1, 3-6, 6-0. "But you see today, he got little bit tight in second set. If I could stay there little more focus, then I think we have some chances to beat him."

World No. 5 Nishikori added that Djokovic, Rafael Nadal, and Roger Federer are all extremely impressive, and that he hopes to emulate their consistency.

"They win couple times a year in a row all the Grand Slams," he said. "Even for me, I had so much, like, stress, pressure in U.S. Open, playing seven matches. I couldn't imagine how they handle the pressure and everything, through the whole year. Especially the people staying No. 1, I bet they have a lot of pressure. They have to always fight, not even on the court, but off the court, too. I think I need little more experience to get there.

“But I think also you get more motivation when you get more high ranking. For me also, I was really enjoying this year. So hopefully I can keep going like this next year."