LONDON—Kei Nishikori, who will face world No. 1 Novak Djokovic on Sunday in the ATP World Tour Finals in London, expects a difficult opening match in the round-robin stage.

“That’s going to be the tough, for sure,” Nishikori said. “He’s been playing really well this year. He’s not going to give you any free points. Good serve, very consistent all around. So it’s going to be a tough one for a first match.”

Nishikori upset Djokovic in the semifinals of the 2014 US Open, and also beat him in 2011 in Basel, a year in which Djokovic also won three Grand Slam titles. However, Djokovic is 4-2 overall against Nishikori, beating him in the ATP World Tour Finals a year ago, and this year in Rome.

Djokovic has won 10 titles this season, including three Grand Slams and six ATP Masters 1000s. World No. 8 Nishikori has won three titles this season, two 500s and a 250.

“What he’s done is amazing, his consistency. Every tournament, he does well. That’s not easy,” Nishikori said. “He only lost a few matches this year [78-5]. He stays focused all the time. I think his concentration is amazing. He’s the toughest player to beat right now.”