Murray brilliantly usurped Djokovic as world number one after a stunning second half of 2016 that saw him win 24 consecutive matches and five consecutive titles, including the ATP World Tour Finals for the first time in his career.

Such a feat had seemed unlikely when Djokovic won the Australian Open in January before completing the career Grand Slam with his French Open victory, and the Serbian is sure to be in determined mood to reclaim top spot.

World number five Nishikori expects the duo to once again be the men to beat over the next 12 months but the Japanese says that Federer - who has not played since Wimbledon due to a knee injury - and Nadal, who has been sidelined by a wrist injury, will help shake things up at the top of the rankings.

"I think those two [Murray and Djokovic] are always the toughest players. I don't know how they are going to start the year but I think they are very strong right now," he said. "I think it is going to change a lot because Andy and Novak are still the top two dominating, but Rafa and Roger are coming back and there are a couple of young guys coming through as well, so I think it's going to be a good year."

Nishikori also reflected on a solid personal effort in 2016, which yielded a bronze medal at the Rio Olympics and a run to the US Open semi-finals, where he lost to eventual champion Stan Wawrinka.

"Yeah, I think 2016 was a great year," he said. "I played very consistent and had many wins that I never had and two finals in Masters [tournaments], which was a very good experience, and a bronze medal in the Olympics, so I think it was a great year."