Michael Schumacher could find it hard going when he makes a Formula One comeback with Mercedes next year at the age of 41, former rival Damon Hill warned on Thursday.
"At some point the season is going to get very tough and is going to get unpleasant and it might bring back memories of why you retired in the first place," Britain's 1996 world champion told BBC radio.
"We'll have to see. A season is a long time and it will put Michael through a severe test of his ability."
Hill, who battled with the seven times world champion repeatedly in the mid-1990s, said Schumacher had plenty going for him, with close friend Ross Brawn running the team that won both titles this year as Brawn GP.
Since Schumacher retired in 2006, Max Mosley has also been replaced as head of the governing International Automobile Federation (FIA) by the German's former Ferrari team boss Jean Todt.
"There's going to be a lot of interested viewers watching the sport for that one, because the suspicion was always that Michael got a little bit of favouritism from the FIA," said Hill.
"I think the FIA are going to have to be on their toes to make sure that there is no suspicion of that in the coming season," he added.
Former champion Niki Lauda, who retired in 1979 and then returned in 1982 to win a third title with McLaren in 1984, said rivals would be looking for signs of weakness from the sport's most successful driver.
"Apart from Gilles Villeneuve, it was more a case of the young guys eyeing me warily and thinking 'is the old fart up to the job any longer?'," the Austrian wrote in the Guardian newspaper of his own return.
"And that's what the young guys of today's generation will be thinking about in respect of Michael.
"I think Michael will be competitive but more importantly it will be great for the sport."
Formula One's commercial supremo Bernie Ecclestone agreed: "I think it is fantastic for Formula One that he turned into a `comeback kid'," the Briton told the FIA website.
"But at the same time I also think that it's good for Michael.
"When drivers have put in many years they sometimes lose focus and motivation - probably that was one of the reasons for his retirement in 2006 - but now he has again put all his strength together and has a 100 percent motivation level and that will give us many exciting races in 2010.
"Forget about winning races - I have him on my bill for winning the championship," added Ecclestone, who said Schumacher's Mercedes team mate Nico Rosberg would also benefit from his compatriot's expertise.