Sebastien Loeb starts the Wales Rally GB six points clear of Marcus Gronholm, knowing a top-five finish will seal a fourth consecutive world title.
Loeb drives over the ceremonial start ramp in Cardiff on Thursday
But the extreme conditions of the 17 gravel stages through South Wales can lead to unpredictable results.
Cardiff hosted the ceremonial start on Thursday evening, with the first stage in Port Talbot at 0853 GMT on Friday.
"It looks difficult, but there is still a hope and I will try to do my best," Gronholm told BBC Radio 5 Live.
Another title for Loeb would see him match Tommi Makinen (who won four titles between 1996 and '99) in the record books.
I don't enjoy driving to finish but, given the importance of the stakes, it's only logical
"The most important thing would be to win the champ - not because of the record, but because of the fight we have had this year with Marcus," Loeb told BBC Radio 5 Live.
"It has been a good championship, a big battle every time so I would be very happy to win. The record is not my most important target. For sure, four world titles in a row would be great, but we will see."
He said Gronholm's crash in the Rally of Ireland had been a critical moment in the season's racing.
"In the end, we picked up an important win which has enabled us to reverse the trend in the drivers' championship and top the provisional standings going into the Wales Rally GB," said Loeb.
"We lead by six points, so we won't need to win at all costs. Should Marcus win, we will have to finish in at least fifth place to make sure of the title.
"That would put us equal on points but we have more wins to our name. We will consequently need to find a pace that permits us to finish inside the top five.
"I don't enjoy driving to finish but, given the importance of the stakes, it's only logical."
Double world champion Gronholm, 39, hopes to emulate Finnish compatriot Kimi Raikkonen in taking a motorsport world title in what will be his 150th race and his last one before retiring.
"It will be difficult to win the title but I have a chance and have to make the most of that opportunity," he said.
"What happened in Formula One is a good example. With two races left, Kimi Raikkonen had a big gap to make up to win the title - and succeeded.
"That's a big spur for me because for Finland to win both the Formula One and the WRC titles would be something special for my country.
"My goal was to win the title in my last year - it's still possible, but I will retire at the top which is what I wanted to do."
Gronholm has a good record at the Wales Rally GB, but he said he would not be taking anything for granted.
"I like the GB Rally, but that doesn't mean that it's easy," he said.
"It's quite difficult in mud, fog and rainy conditions so we have to be a little bit careful so we don't make any big mistakes so we will know much more on Sunday."