The fight for the Harry Vardon Trophy comes down to the last day of the season, as world number one McIlroy narrowly leads US Open champion Fitzpatrick in the projected standings following a fine third-round 65 in Dubai.
But, with Fitzpatrick just one shot off the tournament lead held by their Ryder Cup team-mate Rahm with 18 holes remaining, there is everything to play for on Sunday.
Rahm's bogey-free 65 took the two-time former champion to the top of the leaderboard on 15 under par.
Fitzpatrick, also looking for his third win at Jumeirah Golf Estates' Earth Course, was a shot further back in second following his 70.
Swede Alex Noren sat third on 13 under, with McIlroy and halfway co-leader Tyrrell Hatton tied for fourth on 12 under.
McIlroy came into the season finale at the top of the rankings, but opening rounds of 71 and 68 handed the advantage to Fitzpatrick.
The Northern Irishman hit back on Saturday, though, carding an eagle, seven birdies and two bogeys to climb the leaderboard.
He treated the huge crowds to some exhibition golf, coming close to holing his approach shots at the eighth and ninth before tapping in for birdies there.
McIlroy, who also won the FedEx Cup title on the PGA Tour in 2022, feels landing the Harry Vardon Trophy would be an excellent way to end a fantastic year.
He said: "It's really cool, I've got to this stage in the game, over 15 years as a pro, and I'm still trying to do things for the first time.
"I've never won the FedEx Cup and this tour's rankings in the same year, so it would be really nice.
"It's been a wonderful year. I've played some really, really great golf and really consistent golf.
"If I'm able to go out there tomorrow and shoot a good score and get the job done, it would be a really nice way to end what's been a great year."
Fitzpatrick is aiming to be crowned European number one for the first time in his career and admitted it would be a special achievement.
"It would mean the world," he said. "I'm going to need a lot of things go my way.
"This could potentially, with what happens tomorrow, be a third win here and, if it happens, it could be all three times I've not won the rankings.
"So, yeah, maybe I guess I just leave it a little bit too late to play well, but I've given myself a chance tomorrow, and that's all you can do at the start of the week. I'll do the best I can and fingers crossed."
Englishman Hatton had looked like running away with things in the early stages of the third round, having fired three birdies in his first seven holes to lead by three shots.
But the situation at the top of the leaderboard changed completely on the back nine as Hatton's four bogeys in six holes from the 10th opened the door for the chasing pack.
And Rahm took full advantage, following up four birdies on the front side, including three in a row from the fifth, with three more gains on the back nine to lead by one.