Elena Vesnina (RUS) vs.  Daria Kasatkina (RUS)
They played once before, last year on clay, and that was won by Vesnina in straight sets. Hampered by back issues late in the first set, it became more of a mental and physical battle for the elder Russian.
Vesnina had a visit from the trainer just before Kasatkina was set to serve out the set for the first time. Did that affect the younger Russian? It probably did as she was promptly broken. Kasatkina had a second opportunity to serve out the first set at 6-5 but once again she could not. She was forced to a tiebreak by some brilliant cross-court backhand shots from her opponent. The tiebreak was also topsy-turvy; both players traded mini-breaks until some brilliant defensive retrieving from Kasatkina won her the last mini-break, which won her the first set.
Playing with an injury does not only distract the player with the injury, it also distracts the opponent. While the injured player is worried about the seriousness of their injury, the opponent is also wondering just how serious is the injury. The end result is that both players are not fully focused on playing tennis and their games become choppy. The injured player fearing they may not be able to stay in the rallies, starts going for winners on every shot. The non-injured player goes away from their game plan, hoping subconsciously that the opponent will crumble.
In the end, the mental effort to stay focused took a toll and Vesnina lost her rhythm and too many errors in the latter stages did her in. Kasatkina made no mistake in serving out the match on her first attempt and won 7-6(5), 6-1. She will have another battle in the semifinals against Garbine Muguruza.