A source tells PEOPLE the on-again, off-again couple met up at the Montage Hotel in Beverly Hills, California, where Bieber is staying for the weekend.
Photos show the two stepping out together in L.A. that same day.
The pair "were friendly at the [hotel] bar" Friday, the source says, "but they didn't act like a couple."
"Justin was in a great mood and serenaded her. She was still a bit pouty and playing hard to get," the source says.
Clips of said serenade, a piano-backed performance of "My Girl" and Bieber's song "Sorry," first surfaced on Instagram Saturday.
The fan who filmed clips of the performance also posted a picture of her and her friends posing with Bieber – writing in the caption that Gomez took the picture.
Both pop stars have recently made supportive comments of the other.
VIDEO: Justin Bieber on Getting Back with Selena Gomez, 'I am Sure it Could Possibly Happen'
"I'm never going to stop loving her," Bieber, 21, told Access Hollywood earlier this month. "I'm never going to stop checking in on her.
"We always respected each other and we still respect each other. ... I think we both gotta do some soul searching ourselves, and figure out ourselves before we can just try to figure out each other."
Asked on Ellen earlier this month if he and Gomez would ever get back together, he said, "We have a lot of history together, so I'm sure, I mean it possibly could happen."
"But," he continued, "I think we're both just on our own journeys of just figuring ourselves out, and I think maybe once we figure ourselves out we could come together to make an awesome duo, or she'll find someone awesome, I'll find someone awesome. I just want her to be happy."
In her Elle October cover story, Gomez, 23, said she will "forever support [Bieber] and love him."
The magazine asked, Will they get back together?
"I don't know!" Gomez said.