The video and photo sums it up but let me just ask you whether you think it was worth it for Roger to give up his squeaky clean image by hitting underarm serves against Nadal. And I am talking about putting even more spin on the ball than Gulbis did in this video. I was a player myself who beat ATP ranked players in futures, and I know you can hit it even shorter and with much more spin than Gulbis did. If Fed did that to Nadal he would win the point every time Nadal stands that far back, mess with Nadal's rhythm, and intimidate him. So why did he not do it? It is 100% legal.