The Knight probably would win. OP said "At the top of their game" so I take it to mean the Knight would be in top-tier plate armor, with a shield and longsword. Samurais didn't have shields like that and had far less armor. The metal folding thing isn't an advantage either, they had to do that because iron in Japan was so shit-tier they need to fold it in order for it to be worth a damn. Knight wouldn't need to exploit gaps in the joints, he would just shield bash him and then bring his long sword down on his fucking skull and the samurai could do nothing about it. Samurai armor was designed to protect against arrows, not heavy swords. Knights were surprisingly spry in their plate, known to do cartwheels and flips to show off how well trained they were. You weren't considered worth a damn as a knight unless you could jump onto your horse from the ground. Remember knights had been training in their craft since they were old enough to hold a sword.
If truly at the top of their game, both should be mounted on horseback too, which would give the knight an even bigger advantage because they had better horses with armor, and long lances designed to kill enemy horses or their rider. The Samurai would most likely have a Yumi bow which I don't think would be enough to punch through heavy plate, and a spear which wouldn't be as long as the knight's lance.