(REUTERS) - Novak Djokovic opened the defence of his US Open title with a 6-3, 5-7, 6-2, 6-1 win over Poland's Jerzy Janowicz on Monday, but the laboured performance gave rise to fresh concerns about the world No. 1's fitness.
After a sizzling start to the season that brought Grand Slam wins No. 11 and 12 at the Australian and French Opens, Djokovic's form has plummeted, with a third-round loss to Sam Querrey at Wimbledon followed by a first-round exit at the Rio Olympics.
The Serb might have been in trouble on another day at Flushing Meadows but Janowicz, ranked 246 places below Djokovic, has advanced from the first round just once in four previous US Open visits and looked unlikely to do it again on Monday.
Djokovic, a US Open finalist five of the last six years, next faces Czech Jiri Vesely, a five-set winner over Indian qualifier Saketh Myneni.
Djokovic arrived in New York having been hampered by a left-wrist injury and distracted by undisclosed "private matters" and on Monday trainers were called out early in the opening set to work on his right forearm.
Several times during the match, Djokovic could be seen grimacing when hitting his powerful forehand, while his serve rarely looked threatening.
"It was just prevention, it's all good," Djokovic told reporters.
"Look, each day presents us some kind of challenges that we need to accept and overcome.
"After all I've been through in the last couple of weeks it's pleasing to finish the match and win it."
Up 3-2 in the opening set, Djokovic called for a medical time-out during the changeover as Janowicz took a seat in the stands while the world No. 1 had his forearm massaged.
When play resumed, Djokovic immediately broke the Pole and held serve on way to taking the first set.
But in the second frame Djokovic's discomfort became even more evident as he double faulted and then sent a wild forehand long as Janowicz broke to take control and level the match.
Normal service was resumed in the third and Djokovic moved in for the knockout punch, breaking Janowicz to open the fourth before wrapping up the match.