Here are some examples to demonstrate it:
Zoro vs Cabaji
Buggy had previously stabbed Zoro, and the wounds he received had been hindering him. He went on to stab himself to further restrain his combat ability. Cabaji commented that his wounds were grievous and would be an acceptable excuse to explain Zoro's loss.
Zoro vs the Nyaban Brothers
Zoro only had one sword at the time and this severely hindered his combat ability. He repeatedly wished he had more swords, and when he finally recovered his remaining two swords, he hyped up the power of his Santoryu style.
Zoro at Arlong Park
Zoro was very weakened due to the damage he took from Mihawk. He's shown sweating profusely and short of breath, and ends up falling down a few times during his battle with Hachi. That he has a fever is mentioned several times and we are outright told that the his wounds were too deep and would have killed normal people or at least crippled them for half a year. Arlong questions how he can even stand.
Zoro at the Davy Back Fight
Zoro didn't have access to his swords. Sanji commented that a swordsman without his swords was "pathetic". If an analogy is to be made between Killer with scythes and Zoro without his swords, then something like the above is what I would expect.
Strawhats at Water Seven
The Strawhats had previously been unable to perform at their best as they were emotionally compromised due to Robin's betrayal. Nami mentioned this and that they were now much stronger because they were fighting to save Robin.
Zoro against Ryuma
Zoro only had two swords. He mentioned that he was uncomfortable without his third sword, and lamented being unable to show it to Ryuma. Ryuma himself expressed the desire to see Zoro's Santoryu and commended him for his strength.
Ryuma vs Zoro
Ryuma was fighting with Brook's shadow, and we are told that he was even stronger in his prime, with Ryuma lamenting his defeat and Zoro offering to pretend the fight never happened.
Zoro at Sabaody
Zoro was still suffering from taking Luffy's pain. That his injuries affected him was noted by other Strawhats, Zoro and even Borsalino.
Whitebeard at Marineford
Whitebeard at Marineford had been substantially weakened due to age and illness. His decline was noted by many people: Mihawk, Crocodile, Marco, Sakazuki, Blackbeard and even Whitebeard himself.
Sanji at Punk Hazard
Sanji's body suffered damage while Nami was in possession of it and this was noted by Sanji near immediately.
Smoker vs Vergo
Smoker wasn't fighting at his best against Vergo as he was trying to recover Law's heart. During the fight, Vergo commented that Smoker was fighting stupidly, and after Smoker returned Law's heart Law mentioned that Smoker wouldn't allow himself to be indebted to Pirates.
Doflamingo vs Luffy
Doflamingo had been weakened by Law's Gamma Knife and this was noted by Luffy.
Luffy vs Doflamingo
Luffy had taken some damage in his prior fights with Bellamy and co, and Doffy mentioned it.
Luffy vs Sanji
Luffy had taken a lot of damage in his prior fight with Cracker and Nami mentioned this.
Luffy vs Katakuri
When Luffy was unable to face Katakuri because he could not use haki, he mentioned it of his own accord.
Luffy at Udon
Luffy had been weakened due to the Kairouseki handcuffs.
The above list is by no means exhaustive (for one there's a hard image limit of 40 images and preparing all the panels is a chore). For another trying to reread all 981 chapters of One Piece to identify all instances of characters being weakened or not fighting at their best is a highly nontrivial task. As space permits though, I would add other examples I learn of to this post.
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