Gael Monfils is known for his powerful ground strokes and athletic ability. Here he combines the two to strike an insane winner despite having to contort his body in a way that probably isn't conducive to hitting a good shot. The camera focuses only on Monfils' side of the court making the nature of the winner even more mysterious.
You really need to see the full replay to make an informed assessment, but this was definitely the best shot ever played in a tennis match.
h/t bustedracquet