Just as in chess, a fight match may have any number of possible openers (these are specific to competitions), but may also be found in self-defense scenarios. Each opener has a different sort of strategy involved.
This is otherwise known as "feeling out the opponent." It takes a less aggressive approach, as in this phase, especially in the first round of match-up, fighters for the most part do not know what to expect, how their opponent will react to different stimuli, etc. A fighter who is feeling out the other will often throw test strikes and fakes to gauge the reaction of the opponent. A less experienced fighter may do this out of insecurity or fear; they do not want to make a costly mistake. A more experienced fighter may use this type of opening to "download" the opponent's timing, rhythms, reactions, etc. They also may be a more reactive type of fighter, looking to counter or waiting for the opponent to commit a mistake. A good response to this is to simply force the action and not allow the opponent the room to breathe that they want. A blitz opening is also a good counter to this type of fighter, as it will likely catch them off-guard, especially if they are investigative out of fear.
This is sort of a variation off the investigative opening, the only real difference is that in this, one fighter is simply waiting for the right moment to react or seize an advantage. They are not "investigating" but waiting. Reactive fighters may prefer this approach, so the key here is to disrupt their ability to react properly. Use varied timing or force them to be more proactive with draws.
This is when a fighter comes out and immediately gets right to work, and the pace may vary from slow and steady to them pushing the action at a higher intensity. This is typically done by a more confident fighter. They don't want to waste time dilly-dallying. This also forces the opponent to play at your pace or retreat. A more experienced fighter will still be able to size up the opponent's reactions and techniques from a more intense pace. One strategy to combat this is to change the pace and keep them from controlling the fight. The slow and steady paced fighter is considerate of their stamina; they want to be able to go to the distance. Force them to expend energy they are reluctant to waste.
Sometimes an opponent comes out swinging for the fences. They want to overwhelm the opponent with a flurry of strikes and takedown attempts. This may create an early advantage, especially against a less experienced opponent. Some fighters are simply hoping for a quick win, and this may also be a tactic of a less confident fighter. The drawback to this opening is if the blitz doesn't have the desired effect, a lot of energy is expended. A more experienced or confident fighter can withstand a blitz and tire out the opponent or force them into a different pace. If an opponent opens with a blitz of strikes and no takedown attempts, this may signal that they prefer striking, and might be less comfortable on the ground. Opposite of this, a fighter that is better on the ground may attempt early takedowns, therefore one should attempt to force the standup game.
A surprise opening is typically an ambush, and this doesn't apply to competition. These are deadly. Adversaries will spring an attack on an unsuspecting victim, often from a place of advantage, shadows, from behind, or with multiple attackers. Many times they will use a weapon. Attackers relying on this advantage also typically go for the quick incapacitation or kill, so defending against these is almost exclusively a matter of recognition, awareness, and avoidance. Prevention is the key to survival here.