After he won a maiden French Open to complete the career

Grand Slam, all talk was of the Serb going on to defend his Wimbledon and US Open titles and clinch a calendar Grand Slam.

- Djokovic burden -

That's a feat so rare that only two men have ever achieved it with Rod Laver the most recent in 1969.

The expectations proved too heavy a burden when the 12-time major winner was dumped out of Wimbledon in the third round for his earliest loss at a major in seven years.

Although he then won a record 30th Masters trophy in Toronto, a shock first-round defeat at the Olympics to Juan Martin del Potro and a withdrawal from Cincinnati with a wrist injury suggested all is not well with the 2011 and 2015 US Open winner.

"I am not 100 percent. Hopefully on Monday, when it all starts, I will be there," said Djokovic who faces big-serving Jerzy Janowicz of Poland in his opener. 

"The wrist has not been ideal for three weeks. There are different methods of healing. One involved electricity treatment to enhance the recovery process."

Outside of the top two, five-time champion Roger Federer, who has played every US Open since 2000, called time on his season after a five-set semi-final defeat to Raonic at Wimbledon. The 35-year-old aggravated a knee injury in that loss.

Fellow old-stager Rafael Nadal, the 2010 and 2013 US Open champion, won gold in doubles at the Rio Olympics but lost the
singles bronze medal play-off to Kei Nishikori.

The Games were his first outing since an injury-enforced early withdrawal from the French Open and the punishing schedule of 13 matches in 12 days in Brazil and then Cincinnati caught up with him when he lost in the third round in the American city.

Nadal tackles Denis Istomin of Uzbekistan first up.

Other contenders are likely to be world number three Stan Wawrinka, a two-time semi-finalist, sixth-ranked Raonic, who has yet to get past the last 16, and world number seven Kei Nishikori, the 2014 runner-up.

Cilic, the surprise 2014 champion and 2009 winner Del Potro, the runner-up at the Olympics, should also not be discounted.