Whose ball do they use in foursomes?

I'm guessing that it's the player who tees off, but I'm not sure. It's probably not a big deal for most golfers, since a lot of them use the Titleist Pro V series, but what about Tiger and Nike? I haven't noticed in the President's cup yet, but I bet Nike would be ticked if there was a good shot of Tiger holing a putt with a Titleist. Anybody know anything about this?