Great Britain Davis Cup captain Leon Smith says nobody can question Andy Murray's commitment to the cause as they prepare for a quarter-final with Serbia.

Murray won all 11 of his rubbers in 2015 as GB lifted the trophy for the first time in 79 years and victory over Japan in the first round this time set up a last-eight meeting with Novak Djokovic's Serbia.

Both Murray and Djokovic have busy schedules ahead, with the clay and grass-court seasons leading up to the quarter-final in mid-July, and the Olympics in Rio to come thereafter.

As a result, neither has confirmed whether they intend to play for their nations, but Smith says Murray's commitment to GB thus far has been there for all to see.

"What's remarkable, if you look back at last year, is Andy's commitment to the team," he told Omnisport.

"Singles, doubles, fighting for every point. We win the title and then this year, playing Japan, he'd just lost the Australian Open final and had the birth of his first child and you'd think there was good reason for him to maybe say 'Look, can I skip this one?' – Not at all. He turns out for his country and battles for almost five hours to beat [Kei] Nishikori. 

"That shows you the commitment and enjoyment he has. It's a huge motivational environment. It's a unique atmosphere, only Davis Cup can bring that.

"It's a brilliant tournament and if health and fitness allows it you'll see Andy playing for many, many more years I'm sure."

And Smith says it is simply far too early for either Murray or Djokovic to give a definitive answer on their participation.

"There's a huge amount of tennis to be played between now and then, and you can't think too far ahead. Anything can happen in sport with niggles et cetera," he added.

"We have Andy Murray and they have Novak Djokovic, the whole world wants to see both the best players in the world face off in what is an amazing atmosphere.

"But the reality is we've got clay-court season right now, the French Open very swiftly and then onto grass-court season then four, five days later we're expecting these players to switch again onto clay and play best-of-five matches very quickly and then go to play in the Olympics.

"We can't look too far ahead, Andy's said if he's fit and healthy, then he plays. I have no idea if that's the same for Novak."