Root has been England’s sheet anchor, posting half-centuries in both matches, with his back-to-the-wall 75 in Saturday's second ODI laying the foundation for a five-wicket victory.
"Joe's unbelievable, the way he batted the other night showed his class in these conditions when the pitch is turning," England spinner Liam Dawson told reporters in Colombo on Monday.
"To be that successful and dominate world cricket for the number of years he's played is phenomenal. He's obviously going to go down as one of the greats of the game."
Root has taken the old-school route, at a time when most modern batters try to bludgeon spin into submission by sweeping, reverse-sweeping and going aerial.
He does use the sweep, but he is far from a one-trick pony.
Playing late, using soft hands and milking the bowling for singles, he rotates strike effortlessly, shifting pressure back onto the bowlers.
Sri Lanka know the equation only too well.
"Root has always been a thorn in our flesh and we have seen how, when we dismiss him early, England struggle," captain Charith Asalanka said.
"He holds one end up, builds partnerships and he is England's key, we know that. His experience, with over 22,000 international runs, makes him a huge challenge for us, but we are ready for him in the series decider."
England's growing comfort with spin was underlined in the second ODI.
They rolled out six slow bowling options and sent down 40.3 overs of spin, the most by an England side in ODI history.
Dawson believes that depth could be a trump card with the T20 World Cup looming, which Sri Lanka will co-host with India.
"With the World Cup being in this part of the world, having so many spin options is a good thing for the captain," Dawson said.
"On flatter surfaces, you have to vary your pace and things like that, but in these conditions, you just have to land it in a good spot - and let the pitch do the rest."
The Sri Lanka tour is a dress rehearsal for the World Cup for England.
"This experience of playing here before the World Cup is important for us," Dawson said.
"Hopefully we can win the last game, build some confidence, compete well in the T20 series coming up and head into the World Cup on a high."