McIlroy, who is bidding to emulate Henrik Stenson (2013) by winning the PGA Tour's FedEx Cup and topping the DP World Tour's money list in the same season, leads Fox by 128 points ahead of the season-finale in Dubai.
A victory for either man in the DP World Tour Championship - an event McIlroy has won twice - would guarantee they finish top of the rankings, while US Open champion Matt Fitzpatrick needs at least a top-two finish and help from McIlroy and Fox.
Tommy Fleetwood, Viktor Hovland, Shane Lowry and Adrian Meronk could also lift the Harry Vardon Trophy on Sunday, but they need a win and other results to go their way.
"It's a bit surreal to be honest," said Fox, who has won twice this season and would have overtaken McIlroy with a third victory in last week's Nedbank Golf Challenge, where he finished runner-up to Fleetwood.
"If you'd have said I'd be in this position at the start of the year I probably wouldn't have believed you. It's been an amazing season and hopefully I can finish it off this week.
"But regardless of what happens, I'm really proud of what I've done this year. It's been a fun ride.
"I'm not trying to think about it too much. Rory is the best player in the world. He's obviously the favourite but to be in this position is great and I've got nothing to lose this week.
"I won't do anything different from what I've been doing all year. Just go out and try to beat the golf course and see what happens in that sense and that's served me pretty well and hopefully I can do the same thing this week."
World number one McIlroy, who won the money list title in 2012, 2014 and 2015, said: "It's a nice feeling to come back here and have a chance to win the season title.
"It's been a few years but with my consistent play this year in the majors and some of the bigger events I find myself in a nice position. So yeah, nice to come in with not just one but two things at stake."
The action at the DP World Tour Championship got under way early on Thursday morning, though Fox and McIlroy were only due to head out later in the day.