The prolific Australia captain took his series aggregate to a remarkable 604 runs at an average of 151 with his third century in four matches - including a career-best 239 - as the hosts closed out a draw to retain their 3-0 lead on a deathly dull pitch at the MCG.
Smith, who finished unbeaten on 102 out of 263-4 declared this time, has faced 1,258 balls - or almost 210 overs - and been out only four times since the series began in Brisbane last month.
The world’s No 1 batsman has finished the calendar year as the top Test runscorer too - and for good measure after his latest century, warned he is out to start 2018 with further improvement in the final Test in Sydney next week.
He and David Warner (86) did most to foil England on the final day in Melbourne as Australia comfortably achieved the stalemate, despite having conceded a first-innings deficit of 164 - largely because of an unbeaten 244 from the tourists’ back-to-form man-of-the-match Alastair Cook.
A drop-in surface devoid of pace and bounce, which attracted stinging criticism from all sides - including Smith himself - will be the subject of a review of preparation methods to try to avoid a repeat.
Smith was near-on immovable in the first and third Tests too, yet asked if England must still believe they can dismiss him, Root said: “We have done. Especially when it moves, we are more than capable of doing it.
“We’ve got to make sure we keep trying every option - and if it is a good surface to bat on, keep trusting what we do.”
It is nonetheless ominous that Smith retains an appetite to add to his Bradman-esque returns, and was disappointed he eventually had to stop batting at the MCG.
He had just half-a-smile on his face as he said: “It was a shame we had to call it off in the last hour there ... I could have had that other hour out there!
“I feel like my game is in really good order - I’m adapting to each of the bowlers, changing my plans to them and, how(ever) they’re trying to get me out, I’m making sure I’m in front of the game as much as possible.
“I hope I can keep getting better ... (and) end the series really well in Sydney as well.”
England will head there with the knowledge at least that they can no longer be beaten 5-0, giving Root belated cause for a little optimism.
“I would never question the character of our squad,” he said. “The way we apply ourselves, the attitude we take is always there.
“We are a better team than we have shown on this trip, and that’s a benchmark for us to move forward.”
Reflecting on their improvement, he added: “I’m very proud of the way we went about it. To come off three very difficult games and put in a performance like that is very pleasing - that’s what we are about as a side.
“It’s a fair reflection of what we are capable of as a team. I’m disappointed that we couldn’t quite crack it open today - and we tried absolutely everything.
“We have to try and go one better in Sydney and get that win we want.”