1. There is an ebb and flow to the course of a match. Tennis is unusual in that there is no clock, it ends when someone wins the last point. You can actually win fewer points than your opponent and win the match, this is known as the Simpson's Paradox. What this means is that you can hit a lot of bad shots and still win, so the main thing is to stay the course and not get upset when you don't have the momentum. You can actually feel like you are losing for a large portion of the match and still end up winning! A tennis match could be compared to getting into contract, not all of them will close, problems will happen, it's really important to take it step by step, and tackle issues as they arise without getting too upset.
2. Know your strengths and weaknesses. Know what you are good at, what you need to work on. Know what your opponent can and can't do. If you have a great serve or forehand, this is a weapon. You can get more mileage working on your strengths and turning them into weapons. This might be compared to working in a niche that you are getting many clients from or a particular type of marketing with a great ROI. It might make more sense to really focus in an area that you are getting good results. If you do things that every other agent does, you are turning the business into a commodity, focus on what makes you unique.
3. Leave it on the court and respect your opponent. You will win matches and lose matches, some players are better than you. Try to learn something when you lose, stay humble when you win. I've talked to tennis players that say they can't sleep after they lose a tennis match. Regardless of whether you win or lose, be able to sit down with your opponent and maybe have a beer and laugh and joke afterward. There is a Rudyard Kipling quote at the entrance to center court, sums it up pretty well, I think. Real estate involves conversations with lots of people, great agents, terrible agents, FSBOs, try to learn something from everyone, when you are challenged, see if there is truth to what they are saying.