Spaniard Chacarra began the final round at Delhi Golf Club with a five-shot lead after signing for a 63 when the third round was finally completed on Sunday morning following a weather-disrupted start to the tournament.
And although a fast start saw him extend that advantage to six shots early on, Chacarra ended up having to cling on by his fingernails on the back nine before closing out a two-shot victory over nearest challenger, New Zealand's Daniel Hillier.
Chacarra, who seemed to be showing no ill-effects from a recent fractured toe, finished his round with seven pars to card a 70 which also contained three birdies and a bogey.
He now goes into the final three events of the season leading the race to win the Harry Vardon Trophy ahead of Reed in second and Northern Irishman Rory McIlroy in third.
Chacarra stressed how special it would be to put his name alongside Spanish golfing greats such as Seve Ballesteros and Jon Rahm on the list of winners.
"It would mean the world," he said.
"Those two guys are heroes for me as a Spanish golfer. Seve did everything for European and Spanish golf, and then Jon, he's a guy that I've been looking up to since he went to college and professional golf.
"What he has accomplished, it's amazing. Hopefully, I can put my name on that trophy too. That's one of the goals.
"I know it's not going to be easy. Rory in Dubai always plays well and I know it's going to be a big battle. Patrick too.
"Two of the best players in the world coming for me now, and I know they're not going to make it easy."
Defending champion Tommy Fleetwood finished in a tie for fifth, eight shots behind champion Chacarra on 13 under par, after signing for a final-round 68.
McIlroy is attempting to become an eight-time Race to Dubai winner and knows he will have his work cut out.
"Eugenio is playing well," said McIlroy. "He's overtaken Patrick. That makes my job a bit harder going into the last two events."
The lone Australian to make the cut was Ryan Peake, who ended up in joint-63rd place on one over after a final-round 74.
