Julia Elbaba, the Long Island product who was formerly ranked No. 1 in college tennis her junior year, has been friends with newly minted U.S. Open champion, 19-year-old Bianca Andreescu, for about a year.

Elbaba, whose pro career was interrupted in March when she suffered an ulnar ligament tear was in Andreescu’s suite at Ashe Stadium during her Open Final victory over Serena Williams, sitting with friends and family from Canada.

“It was like cheering against the rest of America,” Elbaba told The Post. “We had to wait for the crowd to stop cheering before we could do our chant. Otherwise it would’ve blended in.”

Before the tournament started, Elbaba and Andreescu held plans to spend a few days after the event in New York. The Oyster Bay resident planned to show her around Manhattan.

The arrival of Andreescu’s massive fame could make this week a whirlwind, though the Canadian teenager spent a quiet Saturday night at the Zuma Japanese restaurant with her parents.

Elbaba, who majored in “Media” at Virginia and has eyes on broadcasting, thinks Andreescu will handle fame well.

“The biggest thing about her is maturity,” said Elbaba, who’s also a diehard Knicks fan. “She’s 19 but just as mature as Serena, even though she’s a teen and Serena’s a mother.”

Asked Saturday night about her new celebrity status, Andreescu said, “I never really thought about being famous. My goals have been to just win as many Grand Slams as possible, become No. 1 in the world. I’m not complaining though.”

Andreescu may have star quality because of a straightforward personality that she showed when breaking down in tears in the interview room, revealing how she had visualized many times beating Williams at the Open.

“My favorite thing about her is no matter how big she’s getting on the tennis court, she remembers where she came from,” Elbaba said. “A very kind person. She speaks really well. Everyone can relate to her and she’s very sincere.”

Before the match, Elbaba hoped Andreescu would use a lot of drop shots against the 37-year-old Serena to get her off the baseline because “she’s so uncomfortable at the net.” But Andreescu didn’t have to. Serena was shaky enough from the backcourt.

Elbaba saw “her tension” in the first game when Serena double-faulted the final two points to get broken. She believes Williams, who has lost her last four Grand Slam Finals, has “a mental block” now on these occasions, being one away from tying Margaret Court’s record of 24 Grand Slams.

“It was clear Andreescu wanted to play Serena’s body,” Elbaba said. “Serena’s so good reaching out with her forehand and she has a good wingspan. If you jam her she doesn’t really know how to respond.”

Elbaba said the scuttlebutt around women’s tennis was if Williams won her 24th here, she was prepared to retire if she captured her 25th at the Australian Open in Court’s home country. Now all bets are off as Williams faces the deepest set of major title contenders in women’s tennis history.

Elbaba is “more motivated than ever” to get back on the tour after watching her friend’s dramatic Open title win.

As she rehabs, Elbaba will be watching every Knicks game, believing in RJ Barrett, who is also form Andreescu’s hometown of Mississauga, Ontario.

“A lot of people were looking at Barrett in the summer league and picking on him,” Elbaba said. “It wasn’t fair. It was just a warmup. I think he’ll shine when the season starts. He was getting nerves out. We need a better season than last year. I don’t want to get the No. 1 draft pick. I just want them to play to win and get after it.”