New Zealand fast bowler Neil Wagner admitted to feeling "a bit shaky" when he struck Australian cricket captain on his head at Hagley Oval yesterday.
Smith, on 78, was hit a solid blow on the back of his helmet by a Wagner short ball in the last over before tea and went down hard in his crease.
The incident was eerily similar to the moment former Australian batsman Phil Hughes was struck on the back of his head in a Sheffield Shield game late in 2014, and died two days later without ever regaining consciousness.
Smith took a couple of minutes to regain his composure before carrying on to make 138.
"Obviously it's not nice," left-armer Wagner said last night.
"It was a bit of a shaky feeling. It's never anyone's intention to try and hit someone in the head and see him go down like that.
"I ran straight up to him. He flashed his eyes a bit but said he was fine.
"We said 'make sure you take enough time and get yourself ready'. He did pretty well from there. A hell of a lot of credit to him, it showed his character."
Australian opener Joe Burns, at the other end, said it was "always a concern when you see someone hit in the head. It got him quite flush as well, which was a concerning part.
"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."
Smith changed his helmet at the interval before carrying on to his 14th test century.
He did pretty well from there. A hell of a lot of credit to him, it showed his character.
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Wagner's two late wickets gave New Zealand some comfort out of a day of heavy toil.
Australia start today just seven runs behind New Zealand's first innings 370.
"It was a bit of luck for us that we'd been trying to get all day and obviously got us back into a bit of a better position at end of the day," he said.
"But with six wickets to get it's still a lot of hard work to do."
The New Zealand players will wear black armbands today to mark the fifth anniversary of the devastating Christchurch earthquake and a minute's silence will be observed at 12.51pm, during the lunch break, when the quake struck.