We don't pay attention very often to the two metal poles that are located right in the middle of the tennis court. They do their job of holding up the net, and since its nearly impossible to hit a ball off one that manages to end up in play, they seem kinda harmless.
Note that I said "nearly" impossible, because Juan Martin Del Potro managed to do it in his first-round match against Guillermo Garcia-Lopez. But just because the ball bounced perfectly to stay in play doesn't mean the point ended up his way:
Delpo's shot obviously woulda ended up out out of play, but Garcia-Lopez nonchalantly slamming home the point for a winner can't have felt good.