Once or twice a year, a tennis player will walk on the court and everything will go right. The first serve goes in and the placement is perfect. The player is seeing the ball extremely well and the contact is solid and consistent. Strategy seems easy and even the simplest ideas are winning points.
However, a player's "normal" day on the court will almost always be a battle to figure out what is going wrong. The best players in the world are experts at solving problems and turning things around if they are struggling. It all starts with accepting that tennis is an imperfect sport. There are going to be unforced errors, miss-hits and bad decisions. Players should try to focus on simple fundamentals (watch the ball, move your feet, expect every ball to come back, etc.), and try to improve their performance as the match goes on.
The goal should be to walk off the court feeling that you got better as the match moved forward and, at the end, you gave yourself a chance to win. A good player can stay close when they are struggling and find a few things that are working when the match is on the line. Again, this is all about executing the basics a little bit better, and not panicking when you have a few mishaps along the way.
Don't dwell on what has already happened when you are in the heat of the match. Always look towards the next shot, the next point, and the next opportunity. If you approach the match like this, you will win quite often, even though you felt that you were having a bad day on the court in the early stages of the match.