Former Australia batsman Damien Martyn believes that Kevin Pietersen is the biggest threat to the Aussies' Ashes hopes.
The England batsman, who has been marred by injury in the run-up to the Ashes series, marked his return to fitness with a century for Surrey last month.
"Pietersen's got that X-factor," BBC Sport quotes Martyn as saying. "He hasn't played much cricket but he's like Shane Warne, he loves the big moments and Ashes series."
The first Test gets underway at Trent Bridge next Wednesday.