It was the day after Rafael Nadal's semi final win over Fernando Verdasco at the 2009 Australian Open that he knew he was in trouble. After playing one of the best matches in recent memory, Nadal could barely walk. His win over Verdasco took over 5 hours long. Now he had 24 hours before the finals against Roger Federer and his legs wouldn't even move. Uncle Toni gave him two options. Learn to suffer and play through it or let the pain overtake him and let Federer walk all over him. Nadal chose suffering as he went on to win in 5 sets.
Without taxing yourself in practice mentally or physically, there will be a point that you are automating your shots and will lose the ability to improve. I see this in matches as well. Let me explain by putting it into context during practices and matches.
Suffering during practice
I see people on the ball machines they bring to the courts to work on their strokes. I admire their perseverance but wonder how much improvement they are actually getting out of it. Most of the time the ball machine feeds right to the player so they do not have to move and it gives them a nice shot right were they like it (usually the forehand). I don't see how this is going to improve one's game as the whole point of tennis is to hit the ball so that it will be hard for the other player to hit it back. Why not set up situations like that in practice? For example, work on positioning yourself away from the feed of the ball so that you have to run to get into position to hit it, often replicating what would happen in a real match. To go further with this, why not make it your weaker side since a good player will attack that side until you miss or give a weaker ball to come in on. In other words, learn to love suffering through a tough practice so you can constantly improve.
To summarize, there needs to be purpose in your practices. Do not just automate hitting and think that you'll get better. It takes a special person to want to put themselves in vulnerable positions to keep improving but if that's what it takes, then why not?
Suffering during matches
This is harder to see during a match but I point it out to my players all the time. The scenario that I see a lot is when someone hits their forehand inside out to the other player's backhand. It's a hard shot and impossible to do much with but just get it back in play. The player who hit the forehand knows this and cheats over to the backhand side a bit to get ready to hit another forehand. Instead of accepting that the player receiving that forehand is on the defensive (meaning they have to just get it back in the middle and have will most likely be running a lot for the rest of the point), they try to hit it close to the line cross court in hopes of avoiding their opponent's forehand. Or they will try to hit the ball down the line to be offensive. Doing this sends a message to their opponent that they are not willing to suffer through a point and play defense for the next few shots. Learn to know when you're in defensive position and play defense. This means more sprinting and reacting than you would otherwise like but accepting it will make you a better player.
Just ask Rafael Nadal.
Learn to pivot with your groundstrokes like this tutorial shows