Can Kei Nishikori take next step and challenge consistently?

Kei Nishikori is ranked fifth in the world

"A win that proved he has reached a different dimension, a testament to his growth."

Kei Nishikori's five-set victory over Andy Murray in the quarter-finals of this year's US Open was one of the best of his career. Probably one of his biggest since reaching the final in New York in 2014.

And it was impressive, coming over four hours and from two sets to one down. But was it, as Japanese tennis veteran Kimiko Date-Krumm suggested, a victory that proved the 26-year-old Nishikori has reached a "different dimension"?

For many the jury is still out.

Not that Nishikori has not been successful already. He has won 11 career titles, an Olympic bronze medal, reached the final of the US Open, made three Masters finals, and hit as high as No 4 in the world. But there has been a common theme that seems to have prevented any further progress: fatigue or injury.

In the 2014 US Open final he was beaten in straight sets by Marin Cilic, saying afterwards that his body was "heavy". After losing in the semi-finals in New York to Stan Wawrinka this year, he said: "I was definitely tired, especially in the end." At Wimbledon he did not look fit from the outset against Cilic and eventually retired at 6-1 5-1 down.

Nishikori beat Andy Murray at this year's US Open

There is no doubting Nishikori's talents, but his progression to the very top has been hampered by consistent injury issues. There is also a question of whether he __can physically go the distance throughout the two weeks of a Grand Slam.

In 2014, one of his coaches, Dante Bottini, said that part of Nishikori's problem with injuries was mental and he "didn't really know how to deal with pain and injury".

It was a clearly a priority for Nishikori and his team at the time.

"If I have one goal for next year that would be for Kei to go out there and play 95 percent of the year healthy," said Nishikori's coach Michael Chang in December 2014. "To go through the year without having an injury or something happen that takes you out for two to three weeks at a time would be a huge accomplishment and that alone would take his tennis to the next level."

Chang also said that he felt Nishikori had an opportunity to win a major on any surface.

That challenge, though, has not materialised. While he has performed well at the US Open, he has never made it beyond the fourth round at Wimbledon, has only made one quarter-final at Roland Garros and never progressed beyond the last eight in Melbourne.

So what does Nishikori need to do to make the next step up?

Perhaps his injuries have just been bad luck. Perhaps he need to lighten his load in 2017 and the years ahead.

"I have been playing a lot of matches this year," he said last week. "Maybe that's why I have been hurting these couple months, not feeling 100 percent yet."

At the moment his body seems as though sometimes it is his toughest opponent.

But because of his style of play that might not change. Nishikori does not have a huge serve - of the eight players in London he has the fewest aces in 2016 - and rather he excels with his court coverage and brilliant backhand.

But grinding out so many points appears to be taking its toll on the world No 5.

Chang admitted earlier this year that "one aspect that could really help him win a Slam would be his serve."

Michael Chang started working with Nishikori ahead of the 2014 season

He added: "It's improved a lot over the past couple of years but it still needs to get better and more consistent."

Nishikori mixed it up at the US Open by serving and volleying, using the tactic to great effect against Murray in the quarter-finals.

Sky Sports pundit Mark Petchey said: "He's definitely trying to protect his serve a bit better this year against obviously great returners."

But will that be enough for Nishikori to consistently compete with the best?

Ahead of the ATP World Tour finals, he acknowledged: "I have to do a little better in the big tournaments, especially the Grand Slams and all the ATP World Tour Masters 1000s.

"Physically, I got much stronger the last couple of years. I try to work hard every day, try to be confident in every match and enjoy my tennis. It's good motivation."

Is motivation enough to see the Japanese make the next step? That remains to be seen.

Check our game-by-game coverage from all group matches at the ATP World Tour Finals in London - including Andy Murray - on skysports.com/tennis, our app for mobile devices and iPad and our Twitter account @skysportstennis.