If we assume that the precognition is perfect, both fighters will know exactly what's going to happen in the window of the precognition, including their own moves. So assuming the fighters have exactly the same window, the actual fight, where and when the decisions are made, would be happening a few seconds into the future. So the fight would be essentially the same as a regular fight.
If the precognitive window is different for the two fighters, the one with the longer window would not likely ever lose, because effectively they have precognition and their opponent doesn't. A skilled fighter can predict or guess their opponents moves without precognition and thus might be able to compensate for a small difference up to maybe 0.1s to 0.5s, especially if the other one is much less skilled.
A fighter might know that they are about to lose. Since a person would react to the knowledge that they are going to be hit for certain in a few seconds, the fighter would foresee that they are going to be reacting to the hit, before they actually foresee the hit itself. A really good fighter should be able to tell how the fight is going to end before it starts by seeing how they themselves will react in the near future to reactions to reactions to reactions, etc. to events in the future. Maybe this is a skill that takes years to master?
With less than perfect precognition, where the fighter sees possible futures, it's not much different from real fights, where fighters constantly try to predict their opponents moves. A big difference is that a few seconds is a long time in a melee fight so there might be a considerable cognitive load on a fighter trying to figure out what to do. Maybe the fighters would focus on only the near 0.5s or so, unless they foresee highly likely win or loss.
