Australian Open: Kei Nishikori has ‘learned to love the gym’, according to Andy Murray

With temperatures in Melbourne consistently hitting over 35 degrees Celsius in recent days, player fitness is becoming more of a factor.

Murray himself was given a straightforward passage into the last eight when his fourth-round opponent Mikhail Kukushkin had to retire at 6-1 6-1 1-0 down having struggled from the off with a hip flexor problem.

Kukushkin had failed to recover from a tough five-setter against Gael Monfils in round three and although Nishikori was also pushed all the way by Jo-Wilfried Tsonga in his last match, Murray is not anticipating an easy ride against the 22-year-old having witnessed his off-season toil first hand.

He said: “I have seen him in the gym a lot, especially in Brisbane the last few weeks.

“I think he has got himself in better shape, he is doing well.”

Murray admitted his former coach Brad Gilbert always thought the Japanese needed to work harder off the court.

“When I worked with Brad and spoke to him about Kei, he said he needed to learn to love the gym,” added Murray.

“And, to be fair, I’ve seen him in there a lot doing good work.

“I think that can then help with your mentality on the court. Because he isn’t as tall as some of the guys, getting stronger will have helped his game. He’s playing very well.”

Nishikori’s defeat of Tsonga moved him into his first grand slam quarter-final and also made him the first Japanese male to make it into the last eight at the Australian Open in 80 years.

And Murray knows he is in for a tough encounter.

“I saw a little bit of his match against Tsonga and he played really, really well,” said the world number four.

“I have practised with him a few times and he’s very good.

“He is very deceptive. For someone that’s not the tallest he creates a lot of power from the back of the court.

“He deals with pace well, he can slice and he moves well.

“He was dictating all the points from the back of the court which is difficult against someone like Tsonga.

“He has also won a few long matches here.”

Murray, bidding to reach a third successive final in Melbourne, is pleased with how his game has progressed since his first-round struggles against American Ryan Harrison.

“Each match (I) got better,” he said.

“I didn’t feel good at all in my first match but I managed to get through and then started serving better, moving better.

“I’ll definitely be fresh for the next few rounds. I just need to make sure I do the right things to get ready for the quarter-final.”

Meanwhile, Andy Murray’s coach Ivan Lendl believes persistence is key to Andy Murray breaking his grand slam duck.

He said: “Part of it is maturing, he is still a fairly young player, he is 25 this year.

“He needs experience and some players learn quicker than others, I was one of the slower ones.

“You just do your thing and keep on doing it and when the door opens you have to step through it.”

Murray has reached three grand slam finals but has yet to win one - failing even to take a set - while Lendl lost his first four before going on to land eight major crowns.

And the 51-year-old points to the quality of the opposition the Scot has faced as another reason not to panic.

He added: “Everyone makes a big thing of Andy being 0-3 in finals but he lost twice to Roger (Federer), arguably the best player of all time and certainly in the open era.

“Losing to Novak (Djokovic) here last year at the time looked like a bad loss but if that had happened at the US Open everyone would have said Novak had had a fantastic year and no-one would have been riding Andy’s tail.”