SARAH MICHELLE GELLAR is enjoying a lovefest with her new TV co-star Robin Williams, but the relationship is rubbing SMG's hubby Freddie Prinze Jr. the WRONG WAY, say insiders.

Williams, 62, is returning to his first full-time TV gig since his career-making sitcom “Mork & Mindy.” He’ll be teaming with 36-year-old Gellar in the CBS comedy “The Crazy Ones,” premiering Sept. 26.

The pair play the father-and-daughter heads of a big-time advertising firm, and Gellar says they are just as close in real life.

“I actually applied for Robin Wil­liams to adopt me,” she quipped.

That’s apparently the kind of thing her hubby of 11 years doesn’t want to hear. And Oscar winner Williams – a married dad of three grown kids – accidentally made matters worse when Prinze, 37, recently visited the set.

“ROBIN WENT ON ABOUT SARAH AND how lucky Freddie is to have her as his wife,” revealed an insider.

“Freddie replied he’s a good husband and father and he likes to think that Sarah is pretty lucky too. Robin was flabbergasted – he didn’t know how to take it.”

At the root of the problem is Sarah’s hero-worship, noted another source.

“Sarah goes on and on about what a dream it is to work with Robin and how he even gives her parenting tips,” explained the source. “Freddie feels like, ‘Enough with this guy, already!’”

Despite the problems, good guy Prinze has cast himself as a real-life “Mr. Mom,” taking care of their two young children so his wife can pursue acting.

Said the insider: “Freddie is great to Sarah.”