When the players finally took to the court, Serena was in the same frame of mind as she displayed in her previous matches. There was none of the shrieking and fist pumping and, in fact, she played some awful tennis, making unforced error after unforced error and showing her opponent just how frustrated she was with her game, which would just not come together for her. On the other side of the net, Kerber held her nerve and played her usual powerful leftie game, serving well and sending Serena racing from corner to corner; and running out a 6-4 6-4 winner in just an hour and a quarter on court. She will meet another in form player on a winning streak, Petra Kvitova, in the semis.
It may be that Serena is having trouble motivating herself after winning Wimbledon and the singles and double gold medals at the Olympics in the last few weeks, but nobody should write her off as a potential winner (again) of the US Open which starts on the 27th, as she always brings her A game to the slams. Only time will tell.