No doubt from shooting himself so often in it earlier today.
It was apparently bothering him earlier in the week too, but we all know how some tennis players are about predating their injuries. It's just so stupid though that he didn't withdraw or tank earlier and rest his foot. He's so much better on grass and he made such a fuss to get in too.
From what I saw, easy first set win for Juan Martin. I actually thought that Dodig might give him a bit of a struggle in the first round, but apparently not....and Ivan puts up only minimal resistance in the second set for an easy 6-1 win for Del Potro. Good start for him.