This is part of my answer from What is the maximum number of blocks you can reach/hit a player in PvP?:
In versions 1.8.9 and below it's possible to do something called combo locking where even if both players have 0 ping and perfect aim, you can consistently hit your opponent before they can hit you for in crazy combos. This is because if you get the first hit, the upward knockback forces the opponent into a higher y-level, making the distance from your opponent's head and you longer than the distance between your head and the opponent, because the base of a right triangle is shorter than the hypotenuse. You can get the first knockback hit with a rod and combo lock someone from there.
Combo locking is best performed with Speed II and sprint resetting in order to minimize the amount of time that the opponent's reach distance is within 3 blocks. Sweaty players who know these mechanics and use them may seem like they're cheating with reach hacks from their opponent's point of view.
Another factor that impacts reach is ping: If your ping is somewhat on the higher end, you can occasionally hit opponents while they're outside the usual reach distance of 3 blocks. This is because you're sending and receiving delayed packets from the server. The first hit deals knockback to the player, and while the server still hasn't sent you their movement packets, you can hit them again. They're outside reach distance server-side, but client-side they are still within reach. On servers with a heavier anticheat, players with lower ping will benefit instead due to their hit registration being better. On Hypixel, hit registration is a bit broken due to the anticheat.
These reasons might be why people can combo you so easily. Basically, the only way to counteract it is to just combo them before they combo you or back out of a combo by pressing the s key.
Although, if you're not playing ranked duels but still getting comboed, you will probably want to practice your aim more. The average player won't know how to combo lock and it's likely that you're just missing hits due to bad aim.