In-form Smith setting new benchmark: Cook

Australian skipper Steve Smith is in such rare form that he's redefining what's possible for a batsman to achieve, according to Alastair Cook.

The former England captain returned to form in record-breaking fashion in the drawn Boxing Day Test with an unbeaten 244.

It was the highest score made by any foreign batsman at the MCG, but after Smith steered Australia to safety with his third ton of the series, he earned high praise from Cook.

"He's playing really well and he's played very well over the past four or five years," Cook said.

"Probably when we first saw him we didn't think he would turn out to be the player he has turned out to be and you've got to give him a lot of credit for the way he's worked at his game.

"He sticks to it for an incredibly long period of time.

"He's kind of setting a new benchmark for what can be possible."

Smith took his series tally to 604 runs with the important knock - 219 runs ahead of David Warner, the second highest series run-scorer with 385.

If he scores at least 26 runs in his next innings in Sydney, he will be tied with Sir Garfield Sobers and trailing only Don Bradman as the second-fastest batsman to 6000 Test career runs.

Clearly, England will need to find a way to curb Smith's brilliance if they are to break through for an elusive Ashes Test win in Sydney.

Skipper Joe Root was also impressed with Smith's performance in Melbourne, but is confident England have an effective strategy to combat him.

"It was difficult (to get him out) on that surface but fair play to him he played extremely well," Root said.

"Even when it is a batter-friendly pitch like that, under the pressure of knowing that you've got to bat all day to try to get a draw is really quite difficult.

"(But) we know that, especially when (the ball) moves, that we're more than capable of (getting him out).

"We've just got to make sure that we keep trying every option if he does get in and keep trusting and believing in what we do."