If we assume that Federer plays the USO at full health then I think he'll probably get it by after the USO or very shortly after that, but that could be a big assumption. All in all, Federer's better surfaces and some of Nadal's worst (indoors) are coming so as long as Federer is fit and the injury isn't too serious he should outperform Nadal by enough to grab #1 before the end of the year. This is just an educated guess/prediction however.