Stan Wawrinka is struggling to be fit in time to defend his US Open title later this month.
The world No 4 has not walked on court since losing in the first round at Wimbledon in early July when he was beaten by Daniil Medvedev.
Wawrinka has been suffering from a knee injury and has had to withdraw from the hard court tournaments in Montreal and Cincinnati, ahead of Flushing Meadows.
And the Swiss star took to Facebook to say: "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.
"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."
The US Open, which is the final Grand Slam event of the season, starts in New York on August 28.
Novak Djokovic has already called time on the remainder of his season because of an elbow injury.
We will have the Masters 1000 series in Montreal and Cincinnati covered via our website skysports.com/tennis and you will also be able to watch all the action live on Sky Sports.
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