The fistfightings work as Quick-Time Event sequences.
Every few seconds, in the screen will appear a message saying which key you must press to throw a punch (in the PC version, it would be "W", "A", "S," or "D": the movement keys). If you press the indicated key (or sequence of keys) in time, Geralt will do a successful combo of attacks. If you don't press any key (or press it untimely), the enemy will do a successful combo of attacks. Sucess enough times, and Geralt will win the fight. Fail enough times, and your enemy will win the fight.
https://www.youtube.com/watch?v=qQiUSGHNe9o
Just keep alert for when the message indicating which key (or button if you are playing with a gamepad) you must press pops up. The messages appears at a specefic place of the screen, you if your attention is at the wrong place (looking at the enemy, or at your keyboard or gamepad) it is possible to fail to notice that messages.
In my first fistfight, the first pop-up message appeared so fast that I failed to notice what was saying (and the enemy punched me a few times), but at the second time, I was already keeping attention to that specific place of the screen, so I missed no further messages and won the fight handily.
If my explanation is unclear, look at the following video (the firstfight starts at 00:35) and keep attention to the pop-up messages indicating which key must be pressed. Be aware that the messages appear in different places of the screen based on which key must be pressed. "S" appears over Geralt's right hands. "D" appears behind his enemy. "A" appears over Geralt's right shoulder. "W" appears in front of Geralt's face. So, if you are looking only at his enemy, it is easy to miss any pop-up message that doesn't says "D".
