Steve Smith was said to be in "shock" after being struck on the back of the helmet by a frightful bouncer from Neil Wagner that had the New Zealand team concerned about his welfare.
The Australian captain's 14th Test century was not his finest for fluency and stroke play but seldom has he been put through the wringer like he was by the Black Caps' all-pace attack.
McCullum hits fastest Test hundred
Brendon McCullum put a smile on the face of New Zealand cricket by hitting the fastest Test hundred of all time against Australia on Saturday. Vision courtesy Fox Sports.
Smith had already shown he was worthy of being Australia's Test captain through his mountain of runs, now he has proven he also has the toughness of his predecessors.
While it is not uncommon for batsmen to be hit on the head, the accident that claimed Phillip Hughes's life has changed the way such incidents are viewed.
The skipper, who fell onto his stomach to regather his composure, was assessed by Cricket Australia medical staff and though clearly shaken by the hit was passed fit to play on.
CA introduced stringent concussion guidelines last year that took the decision out of the player's hands. Earlier this month, NSW opener Ed Cowan was withdrawn from a Shield match after being struck on the head.
Smith, who was on 78 at the time, had earlier been left winded by a bumper from Trent Boult that cannoned into his ribs.
"Certainly it's always a concern when you see someone get hit in the head. It got him quite flush as well, which was a concerning part," Joe Burns said.
"But fortunately the medical staff were out there very quickly and they gave him the all clear. I think luckily the tea break was just around the corner.
"So it gave him a chance to sit down for 20 minutes and I guess just let the shock settle."
Wagner paid tribute to Smith for playing so well after such a nasty blow.
"I ran up straight to him and he sort of flashed his eyes a little bit and I was a bit worried at the start," Wagner said.
"[It's] obviously not nice, is it? It's a bit of a shaky feeling. It's never anyone's intention to try and hit someone in the head and see them go down like that so I think a hell of a lot of credit to him to take a blow like that and stand up and bat the way he did shows the character of the bloke that he is.
"Credit to him, he did pretty well from that. Hell of a knock."
Smith was struck at a crucial stage of Australia's innings with the second new ball less than four overs away. The Kiwis would have fancied themselves taking on a new batsman at the crease.
"There was a bit in the wicket and I thought they bowled pretty well," Burns said.
"I thought they bowled well all day to, I guess, contain two set batters as well. The second new ball certainly did a bit as well. It goes to show that there's enough in the wicket if you bowl in good areas for long periods of time."