It does seem like for right now everything is sold out, but there were probably only a certain amount of tickets reserved for the presale customers.
When I went last year, I didn't get my ticket for Ashe for the second Tuesday until I was such Djokovic was going to be playing that day... so.. that was pretty much Sunday or Monday, and I was still able to get an Ashe seat. It wasn't a great ticket, but it wasn't sold out. I've never ordered my tickets very early before, so I can only assume it's a pretty small amount of tickets for the presale. For example, I need to get a ticket for September 1, which is the only day I'm really worried about since I prefer grounds passes, and even grounds passes are "sold out" right now. When tickets open up to the general public, I think there will be seats available again. I've never had a problem before