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Skilful fighting encompasses an individual's ability not only to perform agonistic behaviours accurately and efficiently, but also to choose the most appropriate behaviour or tactic from its repertoire based on the information available. The efficacy of different tactics may depend both on the behaviour of the opponent and on the individual's internal state, for example a specific behaviour may become less effective over time as fatigue sets in. Furthermore, because fights are dynamic interactions, the ability to choose the appropriate tactic will require rapid information processing and may thus be constrained by an individual's cognitive ability. Here, we conducted a three-phase experiment to investigate the effects of reduced performance capacity induced by exposure to hypoxia and decision-making speed on skilful fighting in the European hermit crab, Pagurus bernhardus. During fights attackers attempt to elicit an eviction either by rapping on the defender's shell or by rocking the defender's shell back and forth. By analysing the effect of exposure to hypoxia and decision-making speed on the relative use of these tactics, we show that contrary to our prediction, neither performance capacity nor decision-making speed alone predicted the type of behaviour performed. Rather, these two traits interacted to determine the amount of effort (raps and rocks) attackers needed to exert to elicit an eviction. Furthermore, we show that while neither performance capacity nor decision-making speed predicted skill when fights were analysed as a whole, the effects of these variables became apparent when analysing the change in accuracy as fights progressed, likely due to the accumulating effects of fatigue. • The performance of agonistic behaviours varies in terms of vigour and skill. • Vigour is linked to energetic status, but the underpinnings of skill are unknown. • We explore the effects of performance capacity and decision-making speed on skill. • We measure two aspects of skill, accuracy and appropriateness (choice of tactics). • We find that performance capacity and decision-making speed impact skill over time. [ABSTRACT FROM AUTHOR]