On this week's Mailbag, I respond to your comments: what if a player tests positive before the US Open final or semifinal, why European players have edged American players on tour, will Rafael Nadal beat Novak Djokovic on hard court ever again, is Dominic Thiem an underachiever, Andy Murray vs. Rafael Nadal on grass, plus your hot takes about Juan Martin Del Potro, Stan Wawrinka, Nick Kyrgios and more.