Lady Gaga enjoyed her "first real Marilyn moment" as she serenaded former US president Bill Clinton at a charity concert in Los Angeles.
The Born This Way star was one of several musicians who performed on Saturday night at the Hollywood Bowl during a concert celebrating the 10th anniversary of Mr Clinton's foundation, which has sought to improve global health, strengthen economies worldwide, promote healthier childhoods and protect the environment.
Emerging from an all-white treehouse, Lady Gaga sported a wavy blond hairdo and red lips like Marilyn Monroe, who famously crooned Happy Birthday, Mr President to John F Kennedy in 1962.
Lady Gaga similarly serenaded Mr Clinton and changed several of her lyrics to reference him, including swapping the title of Bad Romance for Bill Romance.
"I thought, 'My God. I get Lady Gaga, and I will have a heart attack celebrating my 65th birthday'," Mr Clinton later said.
Mr Clinton, who turned 65 on August 19 but celebrated his birthday at a posh Hollywood party on Friday night, sat between wife Hillary and daughter Chelsea in the front row throughout the outdoor event.
Other famous attendees included Maria Bello, Ashton Kutcher, Jason Segel, Ellen DeGeneres, Colin Farrell and Chevy Chase.
"I am the only person in history who got to be president and then had a post-presidential birthday party attended by both Lady Gaga and the Secretary of State," Mr Clinton joked on stage. "I want to thank Hillary because we met 40 years ago this year. When I met her, she was already doing the kind of work you see here long before it was cool."
Other performers included Motown legend Stevie Wonder, country star Kenny Chesney, Somali rapper K'Naan, Colombian crooner Juanes and R&B singer Usher.
Usher kicked off his performance with a take on Joe Cocker's rendition of the Beatles classic With A Little Help From My Friends before launching into his hits Yeah and OMG. The R&B singer accidentally split his trouser legs while dancing to reveal his bare left leg. But the wardrobe malfunction did not stop him, and he continued with his routine after brushing off the incident by telling the crowd: "I work hard."