One of the difficult things for the average player ( and many times expert players too) , is to know when to adjust their game plan or their tactics based on what their opponent is doing. Too many times players get overly "freaked out" when an opponent makes a shot that they have no answer for or plays differently than they are used to. A lot of players are too quick to start doing things that they're not good at and completely changing the game they've worked on for a long time in practice, because their opponent hits a few winners against them. Most times a small tweak to what you're doing will suffice or in some cases just stop worrying about it and continue to play your game. I am going to give you some perspective and some ways to deal with this situation.
Everyone has things they are good at. The fact that someone has a shot they can occasionally hit for a winner shouldn't freak you out unless they are trying it a lot and making it all the time. When you play someone good, they are going to make some shots and win points against you. Once you've identified what it is they're good at you need to ask yourself the following questions.
1.) If they continue to make this shot at the rate they are currently making them is it going to cause us to lose? If the answer is no, keep doing what you're doing and focus on all the points they don't hit a winner on. Do the things you're good at better than they do the things they're good at.
2.) Is there a minor adjustment we can make to lessen the damage such as making a slight positional adjustment. Here's an example. They are lobbing very well and although you are in your Hunter-Helper positions, they continue to push you out of position. Instead of giving up the net and both going back, move your entire unit back two steps. The Hunter will be on the service line ( and still go two steps back) and the Helper is now two to three steps behind the service line. We have adjusted and been flexible without abandoning our game plan. We can also only make these adjustments on important points like game point for the opponent. This also holds for shots like alley shots. We play with someone who is very good and accurate at alley shots. We only worry about them on game point against us ( our Hunter hugs the alley). Most of the time if we're winning we assume he will make some of them but not enough to cause us to lose! That's what important.
In any team sport you have to make choices as to what aspects of your opponent's game you are going to focus on taking away from them. You always want to make them beat you with the things they're not good at , rather than they things they are good at. I can give you an infinite number of examples of this but here are just a few you might be familiar with.
1.) In basketball the opposing team is very good at shooting three point shots. We are going to stay right up in their faces even if they occasionally drive by us for a basket.
2.) In football a team is very good at running the football. We put 8 players at the line of scrimmage and dare them to pass. We make them beat us with the weakest part of their game, not the strongest. If they occasionally complete a long pass we don't panic. If they start to pass a lot and are successful we will readjust.
For some reason in tennis, players think they can stop all of their opponents shots by doing nothing!They play a very neutral, non-committal game. They are reacting instead of forcing their opponent to react. They play scared. You have to identify what they are good at and then force them to do something else. it will be very unnerving to them and will empower you. It will be a battle but a battle you can win.
It's important to remember that you are not going to lose a match because your opponent made enough winners to beat you unless you're playing someone three or four levels above you and even then it's unlikely. The problem is when you get tense because someone hits a winner, you start missing shots you can easily make and you lose because of errors. Stay relaxed, acknowledge what they're doing and decide if you need to do anything at all other than be aware and lastly, make sensible, small adjustments to counter their strengths.