Serena is 38 now and obviously doesn't move as well as she used to. Siegemund should use lots of drop shots and lobs that should at the very least frustrate Serena
Siegemund's done her best work on clay but she's shown improvements on hard. Win over Gauff last round is nothing to sneeze at
Maybe Serena is focused just a little bit more on doubles with her good friend Wozniacki
Should be very windy again in Auckland, maybe another factor that can throw Serena off her game