updated 09/30/2013 AT 10:25 AM ET
•originally published 09/30/2013 AT 10:30 AM ET
Yep, it’s official and a source close to the couple tells PEOPLE, “They are very serious.”
But the pair, who met through a mutual friend about six months ago, started off as friends, the source says.
“Along the way,” the source says, “it turned into something else.”
Specifically, the romance bloomed when Chmerkovskiy, 33, took Upton to an intimate dinner at Scalinatella in the Big Apple, to celebrate her 21st birthday on June 10.
“That’s when it really happened,” says the source. “It all really started just after her 21st birthday. They just clicked. They have a lot of chemistry together.”
“They get along great,” adds the source. “They have a lot of fun together.”