Simona Halep (ROU)  vs. Garbiñe Muguruza (ESP) . These two have faced each other 4 times and Muguruza has only lost once; this happened to have been on clay (in 2015). They are facing each other in a major semifinal so of course there is a lot at stake. However, in addition to a berth in the 2018 French Open final, the winner of this match will grab the WTA #1 ranking. Of course, Halep is the current World #1 and is a 2017 French Open finalist so she is trying to defend both of these achievements. Muguruza is a 2-time major champion and won the 2016 French Open title while Halep hasn't won a major title yet but has reached 2 of the last four major finals: 2018 Australian Open and 2017 French Open. Muguruza demolished Maria Sharapova in the previous round, losing only 3 games in just over an hour. Halep was also impressive in her quarterfinal win but in a very different fashion, surviving a 3-hour war of attrition against Angelique Kerber by losing the first set in a tiebreaker and coming back to win the last two sets fairly easily. Muguruza is playing at a very high level, but I believe that Halep has the skills and mental resilience to survive almost anything Muguruza throws at her. PREDICTION: Halep.
Madison Keys (USA)  vs. Sloane Stephens (USA) . This is a reprise of the 2017 U.S. Open final. I don't think very many people would have predicted these two players would become the faces of American tennis after the Williams sisters but it is looking like the Keys-Stephens rivalry will be one that will not only be defining American tennis, but very possibly women's tennis, at the very highest level of competition. Curiously the two have only met twice on tour and Stephens has won both matches. An important feature of the 2017 U.S. Open final was the fact that Madison was completely overwhelmed by the occasion and could not play anywhere near her best tennis and the match turned into a mismatch. I think her coach Lindsay Davenport will be able to prepare her so that this won' happen this time. The problem is that Sloane is simply a better athlete and mover than Madison and this is likely to be rewarded on clay. Madison is the better server and the (much) harder hitter, so if the match was on grass she would have a distinct advantage, but it's not and she doesn't. PREDICTION: Stephens.