Shane Warne has warned Kevin Pietersen, his friend and former Hampshire team-mate, that he still has everything to prove as England captain.
England's hopes of reclaiming the Ashes next year have risen following Pietersen's successful introduction to the captaincy after replacing Michael Vaughan.
Warne believes his pal will continue to impress in the role but stressed that it is too early to get carried away just because England won the final Test against South Africa - having already lost the series - and then four one-day internationals.
"He is the best player in the England team and one of the best batsmen in the world," Warne said.
"Pietersen has confidence. People will follow him and if England keep their momentum up and win the next few Test series they can give Australia a shake and it will be a hell of a contest.
"But if they keep losing series and only winning dead rubbers then Australia will thrash them."
In a just-published book, Shane Warne's Century, the Australian leg-spinner placed Pietersen at number 33 in a list of the best players he has ever competed with or against.
Warne meanwhile has ruled out a return to Hampshire, saying he is too busy juggling corporate responsibilities with playing poker and life with his three kids.