Los Angeles ? John Ritter, a master of sitcom silliness who ruled TV comedy with “Three’s Company” and then found success again 25 years later with “8 Simple Rules … For Dating My Teenage Daughter,” has died of an undetected heart problem. He was 54.

Ritter became ill Thursday while working on his ABC series and underwent surgery at Providence St. Joseph Medical Center in Burbank for a tear in his aorta, a rare medical condition that can hit without warning.

He died shortly after 10 p.m. Thursday, publicist Lisa Kasteler said. He was accompanied by producers, co-workers, his wife and his 23-year-old son, Jason, said Susan Wilcox, his assistant of 22 years.

Ritter’s youngest child, Stella, turned 5 the day he died. His 55th birthday was next Wednesday.

The son of Tex Ritter, a Western film star and country musician, Ritter was an effortless funnyman who — given the chance — could handle drama as well. Friends recalled him as loving and buoyant.

“It’s like there is a big tear in the world’s heart,” actor Henry Winkler told “Entertainment Tonight” Friday. “He was extraordinary in every aspect of his life, especially as a father. His children were there at every moment of his life.”

Winkler, who co-starred with Ritter on Broadway in Neil Simon’s “The Dinner Party,” was to make a guest appearance on the ABC sitcom. He was on the set Thursday for rehearsal when he was told Ritter had taken ill.

No decision had been made Friday about the future of “8 Simple Rules,” which was to begin its second season Sept. 23, an ABC spokesman said. It’s one of the few bright spots in the struggling network’s lineup. Three new episodes had been filmed, and Ritter was working on the fourth when he fell ill.

“I’m shocked and heartbroken and so sad for his family. I cannot find words to express my sorrow — such a great loss to the joy in the world,” Joyce DeWitt, who co-starred with Ritter and Suzanne Somers in ABC’s “Three’s Company,” told “Entertainment Tonight.”

The sitcom, which aired from 1977 to 1984 and brought a new level of risque humor to TV, was the No. 1 comedy in the 1979-80 season and regularly part of the top 10.

Ritter played a handsome but goofy bachelor who hinted he was gay so he could live with his two female roommates without raising eyebrows. Sexual double-entendres were the order of the day.

Behind the scenes, Somers’ money demands led to clashes with Ritter and DeWitt, and she was eventually written off the show. In a tearful interview Friday on Fox’s “Good Day Live,” Somers said she had reconciled with Ritter at the request of his wife, actress Amy Yasbeck.

She and Ritter had “this phenomenal phone call” in which they worked out all their old differences and had planned to work together again, Somers said. “I learned so much from him. … He was the best physical comic I’ve ever watched.”

Ritter was married from 1977 to 1996 to Nancy Morgan, the mother of his three oldest children. He married Yasbeck in 1999. In addition to son Jason and daughter Stella, he is survived by two other children: Carly and Tyler.