Justin Bieber Surprises Ariana Grande Fans With Performance

Pop star Justin Bieber had quite the Saturday. First he spent the day with his newest little friend, five-year-old Mason Disick, Kourtney Kardashian's son, and he later hit the stage for a surprise performance with singer Ariana Grande.

Kourtney Kardashian posted a super cute picture of her son and Beiber horsing around in a swimming pool on Saturday. Beiber is carrying Mason on his back, sporting sunglasses, a backwards hat, while Mason is all smiles.

The Canadian pop star has been spending lots of quality time with the Kardaishan-Jenner family lately, having appeared in Kendall Jenner’s “Brat Pack” spread in a recent issue of Vogue and has been photographed several times out and about with Kris Jenner.

The momager even reposted Kourtney’s IG pic, writing "#Mason & @JustinBieber family pool day!!! #bestbabysitterintheworld #Repost @justinbieber ???Mason is cool as a fan."

The Biebs followed up his Saturday fun time at the pool with a surprise performance at the American Airlines Arena with Ariana Grande during a Miami stop on her Honeymoon Tour.

Beiber sang the parts originally performed by The Weeknd on Grande’s hit song “Love Me Harder,” but apparently forgot some of the lyrics.

“So I feel bad, I forgot the words, Ariana," he admitted on stage after the song ended. "I've been trying to memorize them backstage for like an hour!"

Grande assured him that “nobody heard,” and the two launched in Bieber’s hit “All That Matters.” The two also performed his 2012 hit “As Long As You Love Me.” Grande stood in for boyfriend, Big Sean, during his verse on the song, rapping his lyrics flawlessly.

Afterwards, Beiber shared a photo of the two on Instagram with a caption that made light of his lyrical flub.

"Had fun with u Ariana, even tho I forgot all the words haha," he wrote.

Had fun with u Ariana, even tho I forgot all the words haha

A photo posted by Justin Bieber (@justinbieber) on Mar 28, 2015 at 8:46pm PDT