This is important. This game is a good workout and how good will you perform in it truly depends on how good you have prepared. Many PVP fights you'll notice that your opponent is breathing hard through his microphone which is a good sign ;)

Moving is essential. Get used to Advanced Locomotion. Press down buttons and move your gloves like you’re punching a speed bag or doggy paddling. Use it to move away from punches quicker than a dodge. Using advanced locomotion is great for escaping away from being cornered. I had experience when my opponent accidentally stepped on his dog and another guy definitely hit a wall with his head. So be aware of your surroundings.
Tip: when you're strafing in the game using locomotion - move in the same direction withing your gaming space in the real life, this way you will be very quick.

Each character has his personal temper. Some of them deal more damage, other ones are able to recover stamina faster and so on. Just point on the character temper description with your controller to read it.

When you opponent has his face open - use left or right cross
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When he blocks his face - throw a hook
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When he has his arms up and protects from a hook or a straight punch - use an uppercut
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You will get an idea.

Back to the Step 2 - Movement. To run towards your body faster you have to swing arms up and down. Some people think they're doing it right but they are still slow. To be faster you don't have to swing further up or down, you actually need to have a better acceleration of each swing if it makes sense. Imagine that you're slashing watermelons with swords in both hands. So increase the speed of lifting each hand.

After hitting too often your character will lose stamina and his left and (or) right glove will be highlighted yellow or red. It means that your punches will be slower and weaker and you need to rest. Keep your hands in blocking position for a bit until your gloves aren't highlighted anymore. Keep in mind, there are some characters who do not restore stamina while blocking.
Speaking of your IRL stamina: do only necessary hits, don't punch after you hear the bell, don't forget to breathe :D