Tennis players use the split step to establish balance before reacting to the opponent's shot. Both feet come off the ground and then hit the ground simultaneously. In the past, I instructed my students to do this right as the player is hitting the ball.
However, the game has gotten faster and faster, with very little time in between hits. This means that the split step must occur prior to the opponent's strike of the ball. If you are at the baseline and have hit a good shot, it is possible to wait until just before your opponent's contact point to split step. If you are inside the baseline or at the net, the split step should occur right as your opponent starts the racquet moving forward.
Keep in mind that if you hit it to the opponent at the net, you need to split step even sooner—I would recommend doing this as your shot is approaching the net. This sounds a little crazy, but you’d rather be stopped and balanced, than moving towards where you think the ball is going to end up.
The exception to this split step timing might be when your shot is not hit well and you have to guess where that next shot is going. Hopefully, this will not happen too often.
Trust your reaction time and your ability to prepare quickly and you will reap the benefits of the perfectly timed split step.