Justin Bieber Parties in Miami While Selena Gomez's New Relationship Heats Up

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Justin Bieber isn't letting the Niall Horan and Selena Gomez romance rumors get to him.

As things between his on-again, off-again love interest and the One Direction heartthrob have been heating up, the 21-year-old pop star has been living it up in Miami, Florida.

WATCH: Selena Gomez Fuels Niall Horan Dating Rumors

A source tells ET that the "Love Yourself" singer checked into Villa Casa Casuarina on Tuesday, reserving nine out of the 10 rooms. He then had two back-to-back nights of partying with five of his close pals and security.

Bieber kicked off the first night of celebrations at Wall Lounge inside the W South Beach Hotel. According to a source, he arrived to the club around midnight, where more than 15 girls were waiting for him at his table.

Biebs was "in an extremely good mood" and "instructed everyone to put their phones down" as he sang a mashup of his current hits, the source said. He left around 4 a.m. but apparently the night didn't end there. "He brought 50 people back to his hotel, including a number of attractive women," the source added. "Everyone partied and drank until 8 a.m."

WATCH: Justin Bieber's Mystery Instagram Girl Found

He was ready to do it all again on Wednesday, this time at LIV nightclub inside the Fontainebleau Hotel. ET has obtained video from inside the club, and in typical Bieber fashion, he got behind the DJ booth and gave another spontaneous performance.

In the video, you can see Bieber -- dressed in an unbuttoned flannel over an extra-large T-shirt, skinny jeans and black cap -- playing Rick Ross' "B.M.F. (Blowin' Money Fast)" on the turn tables. He is then introduced to the stage as fitting lyrics from Meek Mill's "Believe It" track -- "I got that Justin Bieber please believe it" -- played.

WATCH: Selena Gomez and Niall Horan Leave Jenna Dewan-Tatum's Birthday Together

His impromptu performance included three of his biggest hits this year -- "Sorry," "Where Are U Now" and "What Do You Mean?" A source tells us that shortly after singing, he "headed back to his hotel with just his friends."

Meanwhile, Gomez was in Philadelphia to perform at Q102's iHeartRadio Jingle Ball. Hours before the "Good For You" singer took the stage at Wells Fargo Center, she fueled more rumors that she and Horan were a couple by supporting One Direction over Justin Bieber on Instagram.

PHOTOS: Justin Bieber Reunites With Selena Gomez -- See the Pics!

When user @disneylandgomez asked whether she should purchase 1D's Made In the A.M. or Bieber's Purpose album for a friend, the former Disney star commented, "Made In the A.M."

Is it too late now to say Sorry? Hear more in the video below.