Blindsided by ex-hubby Brad Pitt’s stealth marriage to Angelina Jolie, Jennifer Aniston has fast-tracked a quickie wedding to fiancé Justin Theroux, The National ENQUIRER has learned exclusively.

The “Friends” star had been planning a lavish wedding in Greece, but The ENQUIRER has confirmed through inside sources that Jen has decided to tie the knot on the beach in Mexico.

“It will be a laidback, low-key affair in a private home at one of Jen’s favorite destinations – Cabo San Lucas,” a close friend of the couple tells The ENQUIRER.

“Jen wants a small wedding with no more than 20 family members and friends.”

The guest list will include several members of Justin’s family, along with Jen’s father, actor John Aniston, and 78-year-old mother, Nancy Dow.

While their relationship has been stormy, Jen has reconciled with her mom, who suffered a stroke in 2011 and may be developing Alzheimer’s. “Justin was instrumental in getting Jen to bury the hatchet with her mom, and Jen wants her mom there to witness the joy of their wedding day,” said the source.

“Although there will be bold-faced names on the guest list, Jen wants her wedding to be as un-Hollywood as possible.”

The group will go barefoot on the beach for the ceremony, according to sources, with Jen, 45, in a simple sundress and Justin, 43, in a white shirt and khaki slacks.

A beachy reception will feature Mexican fare, sources say.

Jen is considering two private homes to host the affair, sources add. One spot, Casamigos, is co-owned by Brad’s best buddy, George Clooney!

“Jen knows that will really bug Brad,” the insider added. Close friends say the pair picked Cabo because it’s a favorite vacation spot. They rang in the New Year there with pals Jimmy Kimmel and wife Molly McNearney, Howard Stern and wife Beth Ostrosky, and Courteney Cox.

“Jen says a quickie wedding will be a breeze to pull together there because they know it so well,” noted the insider.

Meanwhile, sources say the famously fit actress is working feverishly to look even more fabulous for her big day.

“Jen’s number one goal is to look better than Angelina on her wedding day!” the insider revealed.

“She’s been doing yoga two hours a day, weight-lifting and cardio. She quit eating carbs, and allows herself just one alcoholic drink a week.”

While Justin is thrilled about taking a faster track to the altar, Jen’s competitive obsession with Brangelina upsets “The Leftovers” star, insiders say.

Explained the source: “Justin thinks Jen keeps an unhealthy eye on Angelina and maintains a one-sided competition with her,” the source says. “. He’s hoping that once they make it official, Jen will finally let go of her fixation on Brad and Angie.”