Didn't look like it was his knee, but more his upper leg. He apparently had some (small) problems moving on the court after the first set, and also it was a bit apparent that he's not really a doubles specialist
He had a small tear in the quadriceps since the Miami finals. Was bothering him enough not to use him as a singles player, but well, aggravated a little bit on the doubles match.
Not to take anything from the Swedish team. They certainly were the better doubles team (BY FAAR) and on the single rubbers, they seemed more adapted to the Taraflex court. So kudos to them and congratulations!!
Btw, Peter. This carpet is faster than Shanghai. This is Taraflex, and seemed like even faster than Croatia's. Insanely fast.