By Steve Keating

NEW YORK (Reuters) - Novak Djokovic opened the defense of his U.S. Open title with a 6-3 5-7 6-2 6-1 win over Poland's Jerzy Janowicz on Monday, but the labored performance gave rise to fresh concerns about the world number one'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 U.S. Open visits and looked unlikely to do it again on Monday.

Djokovic, a U.S. 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 two hour, 37 minute match, Djokovic could be seen grimacing when hitting his powerful forehand, while his serve rarely looked threatening, stuck at around 100 mph.

"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 number one 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.

The year's final grand slam got off to a glitzy Hollywood-style start, with a performance from Phil Collins to mark the arrival of the $150 million retractable roof at the stadium.

"It's hard to put on show after Phil Collins," said Djokovic, before breaking into one of Collins' hits during his on court post-match interview. "The U.S. Open is the most entertaining grand slam.

"It was wonderful to come back and play a night session that is undoubtedly the most special night session we have in our sport."

(Editing by Larry Fine/Peter Rutherford)