From unstoppable to start the year, Novak Djokovic suddenly seems somewhat vulnerable heading into his US Open title defence.

The world number one started 2016 with 14 consecutive wins, before adding the elusive French Open crown to his success at the Australian Open.

That kind of incredible dominance was always going to end. And it did.

As talk of a calendar Grand Slam grew louder, Djokovic – bothered by personal issues – suffered a third-round loss at Wimbledon.

He scratchily claimed the title in Toronto to follow, before a first-round loss at the Rio Olympics – this time battling a wrist injury – to Juan Martin del Potro.

While he has moved on from the personal problems, which he was unwilling to detail on Friday, the wrist continues to be an issue.

Yet his impeccable early-season form had fallen away before then.

A tricky draw has followed, with the likes of John Isner and former champion Marin Cilic in his quarter, and Rafael Nadal and Milos Raonic the same half.

Djokovic has won four of the past five slams and the absence of Roger Federer (knee) and wrist woes for Nadal seemingly open up the year's last major.

While it shapes as an opportunity for those outside the 'big four', it is also a huge chance for Andy Murray.

The world number two has happy memories at Flushing Meadows, dating back to his maiden major success in 2012.

Murray carries plenty of momentum to New York, although his 22-match winning run was ended by Cilic in Cincinnati.

The Wimbledon champion defended his gold in Rio and is the man to beat given his recent form.

Despite a seemingly minor shoulder injury and heavy workload, there appear to be few other problems for Murray, who is eyeing a fourth major crown.

However, his record since beating Djokovic in the US Open final four years ago is an ongoing worry.

Since then, Murray has lost 16 of 19 meetings with the Serbian.

He has the dangerous Lukas Rosol in the first round and 2014 runner-up Kei Nishikori is the biggest threat in his quarter, before a potential semi-final with Stan Wawrinka.

But Djokovic's dip is Murray's gain and he seems to be peaking at a good time – mostly, when his rival isn't.