As for physically, he was bleeding and sweating constantly, and he got heart attack during the fight, that also weakens him physically not only in terms of Haki. If your physical and Haki stats are weaker, your capacity of using the DF gets weaker as well, that's how its been so far.
As I said MF Whitebeard wasn't on Yonko level. He was so weak that Marco and Jozu needed to save him several times. It should've been the opposite, Whitebeard should've saved Marco and Jozu. Look at Doflamingo, he saved Trebol from Law after Law used Radio Knife on Trebol, even though he was also fighting with Luffy.