Novak Djokovic breaks down in tears during US Open first-round loss to Mariano Navone

Novak Djokovic broke down in tears during a US Open first-round defeat by Argentina's Mariano Navone which provided further evidence age is fast catching up with the 39-year-old great.

The 24-time Grand Slam champion was overcome by emotion in the deciding set of a dramatic match where he struggled physically throughout and had to vomit into a towel bin.

Djokovic, seeded fourth, showed the fight for which he has always been renowned before succumbing to a 7-6 (7-5) 5-7 4-6 6-2 6-1 loss in a humid New York.

It was the first time the Serb had been beaten in the opening round at Flushing Meadows and the first time he had lost in the opening round of a Grand Slam tournament since 2006.

"I think it was obvious that it was an awful feeling on the court for me," Djokovic said.

"I don't want to take too much out of Mariano's win. Congrats to him, he's a nice guy and a fighter - but I didn't enjoy myself."

Djokovic managed to raise a smile as he congratulated 25-year-old Navone on what will rank as one of the biggest victories of the world number 49's career.

But, after showing grace and class while congratulating his opponent, a disconsolate Djokovic was unable to keep a brave face for long and appeared close to tears again as he trudged back to the locker room.

The defeat ended Djokovic's latest bid for a standalone record 25th Grand Slam singles title, which would move him clear of Australia's Margaret Court.

Djokovic had reached at least the semi-finals at six of the previous seven Grand Slams going into the US Open, but the manner in which he struggled in the humid conditions was a worrying sight for his supporters.

Djokovic looked nauseous throughout a tense encounter and he appeared to be dealing with a similar issue to the one which affected him at the Cincinnati Open two weeks ago.

In his only appearance on the North American hard courts before heading to New York, Djokovic wilted in similarly humid conditions in a shock early exit to Argentina's Thiago Tirante.

Afterwards Djokovic revealed he had been hampered by a "health condition" which has been "bothering" him in oppressive conditions over a number of years and, discussing the issue with reporters shortly after losing to Navone, called it a "serious issue".

"It's something that I have been carrying pretty much every match this year - vomiting, throwing up," Djokovic said.

"And then my whole body starts to collapse basically, cramping and pain - the back just gave up in the end and shoulder, and I couldn't close it out."

Djokovic made a promising start as he broke Navone's serve at the first attempt for a 2-0 lead, only to see that advantage wiped out in the seventh game as he began to struggle in the humid conditions.

The catalyst for the former world number one's troubles appeared to be a lengthy rally which sapped him of energy and left him struggling to recover.

A flustered Djokovic seemed to drop speed in both his serve and groundstrokes, calling out the doctor after the ninth game for some tablets to help alleviate what was bothering him.

He continued to look distressed as the opener moved into the tie-break - wearily bowing his head and bending over double between points - as Navone maintained his focus to strike first after one hour and 13 minutes.

As Djokovic toiled, an eerie hush fell around Arthur Ashe Stadium which indicated the crowd's growing concern for the four-time champion.

The physical problems did continue to linger, but Djokovic perked up enough - seemingly helped by the cooler conditions under the roof - to create the only break points of the second set.

After missing chances in the third and fifth games, he dug deep to strike for a 6-5 lead and served out to love as he levelled the match.

Djokovic becoming more animated in the early part of third set was a surefire sign he was starting to feel more like his old self - although not for too long.

With the set locked on serve at 3-3, Djokovic looked unsteady on his feet again before pouncing on a poor service game from Navone to break for 5-4 and holding serve to move into the lead.

These are the positions from which prime Djokovic would usually go on to win, even when he was struggling physically.

The fact he was unable to do that against Navone, who has never gone past the second round in New York, was another indication of his declining powers.

What had been a tight match quickly ran away from Djokovic and became a difficult watch for the thousands still inside Ashe as local time fast approached midnight.

The atmosphere became subdued again as it became clear that Djokovic was not going to be able to wriggle out of trouble on this occasion.