Juan Martin del Potro has basically been off the tour since Indian Wells.  He got wrist surgery and was expected to be off the tour for at least 3-6 months.  del Potro has signed up to play in Thailand, but that is at the end of September.  The US Open starts the last week of August into the first week of September.  He is in the provisional draw.  It is unclear at this point whether del Potro will defend his title.  At this moment, due to winning the US Open and getting to the finals of the ATP World Tour Final, del Potro is still in the top 10, but he would fall out of it if he fails to play in the US Open.