Wasn’t sure exactly how was Obi wan able to keep up with Anakin in ROtS in Mustafir? Was Obi wan amped or powered up some how? Or because he knew Anakins fighting style as he was his master .. What does canon say about this ?
Bascially Anakin was in a very unstable mental condition which rendered his clarity and perception making him less combatively powerful during the fight. In addition to all this, Obi Wan knew Anakin and his fighting style really well to the point that he could instinctively react to Anakin simply based on this. If Obi Wan wasn't Anakin's master for years, he would get stomped just like Dooku.
What was said above. In addition, Obi-Wan is the best defensive fighter in the Order. All he really had to do was stall it out until Anakin did something retarded. Even then, Obi-Wan barely won.
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"The Daemon lied with every breath. It could not help itself but to deceive and dismay, to riddle and ruin. The more we conversed, the closer I drew to one singularly ineluctable fact: I would gain no wisdom here."
Although I don't think Obi Wan being the best defensive fighter was the biggest factor in his ability to hold off Anakin. Obi Wan got slapped by Dooku despite this. The biggest factor that allowed Obi Wan to last against Anakin was his knowledge of Anakin and how Anakin fights allowing Obi Wan to react instinctively. THis and Anakin's mental state were the biggest factors.
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1 Anakin is overconfident and his mental condition is very unstable( As we see him force-chocking Padme whom he wants to save, His mind is a mess during this fight)
2 Obi-Wan knowing Anakin's fighting style too well
3 Obi-Wan is one of the most defensive fighters in SW