I’m a huge Isner fan and have been waiting for his game to progress more beyond his serve. Despite his 7-6/6-4 vistory over Janowicz, staying in rallies enough to win the point against big hitters has been a challenge for him. This was witnessed in the 2015 Hopman Cup. In matches like this one, aside from his serve, Isner is very dependent on his opponent’s errors in order to remain in the game. With janowicz’s hard, flat return it was difficult for a very-tall Isner to get under these balls.
Another issue I’ve seen with Isner, and others that are losing points in the net or going long, is that they are not bending their knees enough in their returns. This is even more problematic with Isner’s height. In most of his forehand returns that were hit in the net or went long, Isner was not bending his knees enough. If I were isner’s trainer I would prescribe him some light weight barbell squats to increase his vascular capacity and increase stamina and movement for bending those knees.