Andy Murray (GBR)  vs. Stan Wawrinka (SUI) . These two over-30 players have played seventeen times before, and the World #1 leads the Word #3 10-7. Interestingly this match is a rematch of last year's semifinal in which I correctly predicted Murray would prevail. However last year Murray was having much more success on the clay and other courts, having won the Rome title. This year, neither player has been playing that well but Wawrinka is yet to drop a set at Roland Garros while Murray has dropped 3 but never more than one in any match so he has not been in danger of losing. That being said, Wawrinka has already won this tournament (d. Djokovic in 2015 French Open) and leads 2-1 in their clay meetings while Murray leads 3-2 in 5-set matches. I would be shocked if this match does not go to 4 if not 5 sets, if it doesn't then almost certainly Wawrinka would have bashed Murray off the court. That is unlikely to happen, but I believe the end result will be the same, leading to Wawrinka's 4th major final. Mad Professah's pick: Wawrinka.
Rafael Nadal (ESP)  vs. Dominic Thiem (AUT) . This is the popcorn match of the clay court season, the equivalent to the Simona Halep-Elina Svitolina quarterfinal on the women's side between the two players with the best clay court records this year. Thiem is in his second consecutive Roland Garros semifinal, after dispatching the defending French Open champion in surprisingly easy fashion without dropping a set against Novak Djokovic. There is no question Nadal is the best clay court player of all time, and now sports an unbelievable 77-2 record at the French Open. His two losses came in 2009 to Robin Soderling and 2015 to Novak Djokovic. In fact, Nadal losing on clay at any tournament is a rare event. Interestingly, though, Thiem has had 6 meetings with Nadal on the surface and won twice (making him one of the very few active players to have multiple wins over Nadal on clay), including just a few weeks ago in the Rome quarterfinal. It should be noted that beating Nadal in best-of-5 is something that Roger Federer has never done, for example. I doubt it is something that Dominic Thiem will do tomorrow (but it is something I believe he will do in the future). Mad Professah's pick: Nadal.