Reigning US Open champion Stan Wawrinka is battling to be fit for this year's tournament after announcing his withdrawal from the forthcoming events in Montreal and Cincinnati.
Wawrinka struggled with a knee problem during the grasscourt season and lost in the first round of Wimbledon to Russia's Daniil Medvedev.
The world No.4 said he would not return to the court until he was fully fit, and he will miss the next two Masters events on the ATP Tour calendar.
The top players are due to return to action for the first time since Wimbledon at the Rogers Cup in Canada next week, which is immediately followed by the Western & Southern Open.
"After much backwards and forwards and consultations with my doctors and my team, unfortunately I have decided to skip Canada and Cincinnati to be on the safe side, even though I've been battling hard to make these events," Wawrinka said via a statement.
"Hugely disappointing for my fans and myself that I have to make this decision, but I need to be 100 per cent confident before I resume competition till the injury that plagued me in Wimbledon has been resolved.
"My team and I are doing everything possible to make this a speedy recovery."
Wawrinka's absence from the two tournaments means that, if he is fit to defend his title in New York, he will almost certainly go into the year's final grand slam having not played a match for nearly two months.
Novak Djokovic is also missing having called an early end to his season to rest his troublesome elbow while world No.1 Andy Murray has been named as the top seed in Montreal but is still deciding whether to play after the hip problems that hampered him at Wimbledon.
Wimbledon champion Roger Federer will definitely be in the field and, along with Rafael Nadal, is looking to close in on Murray's in top spot.
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