Djokovic advances to US Open second round

Novak Djokovic says there is no need to publicly discuss his fitness despite being in pain with a wrist injury during a first round US Open win.

An injured Novak Djokovic has advanced to the second round of the US Open. (AAP)

Novak Djokovic double-faulted, then shook his right arm and grimaced.

Seconds later, a weak serve produced a wince, then was followed by a missed forehand that gave away a set in the defending champion's first-round match at the US Open.

While he managed to emerge with a 6-3 5-7 6-2 6-1 victory over Jerzy Janowicz of Poland, there were plenty of signs of trouble, starting with a visit from a trainer who massaged Djokovic's bothersome arm after only five games.

Asked about his health during an on-court interview, Djokovic deflected the question, saying: "I don't think it's necessary to talk about this now. I'm through. I'm taking it day by day."

During the match, Djokovic hit first serves around 100mph, sometimes slower - 25 mph or so below what's normal for him.

He hit second serves in the low 80s mph. He flexed that right arm, the one he has used to wield a racket on the way to 12 Grand Slam titles, and appeared generally unhappy.

In the stands, Djokovic's coach, Boris Becker, gnawed on his fingernails, looking nervous as can be.

This was the No. 1 ranked Djokovic's first match at a major tournament since losing to Sam Querrey in the third round of Wimbledon, which ended the Serb's bid for a calendar-year Grand Slam after titles at the Australian Open and French Open.

Heading into the US Open, Djokovic spoke about dealing with a left wrist injury that flared up in the days before the Rio Olympics this month.

But that appeared to be just fine against Janowicz, a former top-20 player who reached the semi-finals at Wimbledon in 2013 and is now ranked 247th after his own series of injury issues.