Serious Marco fights Base King and Base Queen. He holds his own in 1 vs 2 pretty well, but can't really damage them. He is serious while they are pretty much holding back (no Hybrid etc).
Then Sanji starts fighting Queen, while Marco presumably continue fighting King.
Now Marco is exhausted.
Logic tells that Marco is exhausted mostly because of King, plus the previous efforts vs King + Queen and healing the samurai.
B-but Marco is exhausted because King + Queen, even if he is fighting King alone since many chapters ago.
Or: Marco is exhausted because Gifters are supporting King. Yes, I can easily see Gifters being so strong that they overcome Marco's regeneration more than playing around King + Queen or Admirals could lol.
King = Marco at the very least.
Most likely, King >= Marco if not > Marco.
And Sanji, lol. He got stomped by King and Queen in seconds. This shows how far is he from YC1 level, as Marco could at least hold his own decently vs King and Queen.