Rugby World Cup winning number 10 Dan Carter says it's the 'x-factor' that puts All Black Beauden Barrett ahead of rival first five-eighths at the Rugby World Cup.

Barrett, Ireland's Johnny Sexton and England's Owen Farrell have been the leading number 10s in the world since Carter retired from international rugby following the final at the 2015 tournament.

Barrett has won World Rugby player of the year twice (2016, 2017) while Sexton took out the award last year.

However, Barrett might not even be wearing the number 10 jersey at the World Cup, having been used at fullback this season with Richie Mo'unga starting at first-five.

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In comparing Barrett, Sexton and Farrell, Carter told the Times what gives the All Blacks star the edge.

"Beauden is the kind of player, whether at fly half or fullback, who has the X factor to change a game with one play. He has lightning speed and can do freakish things that are beyond Johnny and Owen," Carter told the Times.

Carter said there have been questions over Barrett's ability to cope with pressure but he would have learned from kicking errors in crucial tight encounters.

"In tight World Cup matches, your game management needs to be world class and there have been questions over Beauden," Carter said.

"He is a class player with a cool head, who reads the game well, and there are no worries there. He will learn from his setbacks — like against the Lions when he missed some kicks. The closest thing to a World Cup is a Lions series in terms of being able to perform under pressure, so if he is in a similar situation I am confident that he will have learnt a lot."

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