Justin Bieber Rocks Ariana Grande's Concert In Surprise Spot Despite Forgetting A Few Words

Justin Bieber and Ariana Grande's long awaited duet never saw the light of day.

Then, along came three.

That is, if you were at Miami's American Airlines Arena on Saturday for the octave-scaling songstress' Honeymoon Tour. Then you would have seen Justin surprise the crowd, when he joined Ariana onstage to sing The Weeknd's Abel Tesfaye's part on "Love Me Harder."

The two 21-year-olds also partnered on Justin's Believe album hit "As Long As You Love Me" and his Journals single, "All That Matters." As a result, the hashtag "Justin e Ariana" trended on Twitter.

On Twitter, Rixton's Lewi --- the band supporting Ariana --- tweeted about the delirious response to Justin's guest turn-up.

Arianators and Beliebers were on high alert long before the Canadian and the Boca Raton singer hit the stage, after fans spotted Justin arriving at the American Airlines Arena for the soundcheck, and a snippet of Ariana rehearsing "All That Matters" surfaced online.

Shortly before the concert started, Justin tweeted, "U guys have been asking for this for a while. Get ready."

At one point during the show, Justin told the crowd, "Can't believe I'm back on stage. It's been a while." He also forgot some of the lyrics in "Love Me Harder," which he apologized for, profusely, onstage.

But, from the sound of cheers on the night, no one cared.

Justin's excited dancing in "Love Me Harder" was très cute.

The pair's rendition of "As Long As You Love Me" also appeared to go down like a storm with the Miami crowd.

Ariana rapped her boyfriend, Big Sean's, interlude in the Bieber classic.

The Biebs busted some familiar dance moves during the concert.

Justin and Ariana's performance of "All That Matters" was also a crowd-pleaser.

At the end of the surprise, the two superstar pals hugged.

Justin Bieber and Ariana Grande
(Photo: Justin Bieber and Ariana Grande embrace in one of their final moments on stage at Miami's American Airlines Arena on Saturday, March 28)

Both singers took to Twitter to thank each other, and Miami.

Scooter Braun (who manages both singers) also shared effusive tweets and photos after the show.

The Biebs hinted at more in a final tweet, which is --- no doubt --- referring to his expected guest appearance at the Ultra Music Festival to sing "Where Are Ü Now," his collaboration with Jack Ü's Skrillex and Diplo.

Meanwhile, a slew of pictures of the long-dreamed-about duet between Ariana and Justin are currently doing the rounds. Enjoy.

Justin Bieber and Ariana Grandee
(Photo: Ariana Grande was a support act on the second American leg of Justin Bieber's Believe tour in 2013)

A heartfelt moment from the Biebs.

Justin Bieber
(Photo: Justin back in action on stage when he crashed Ariana's Miami stage At the American Airlines Arena)

The "Baby I" singer looked amazing in a sparkly, white leotard dress and skycraper heels, while the Biebs kept it cool in an all black ensemble, after removing a white jacket when he arrived onstage.

Justin Bieber
(Photo: The two singers whipped up the crowd with a performance of three duets)

Justin Bieber
(Photo: If we were making up a caption, which we are - we'd say Ari is thinking 'What are you doing Justin?)

Justin talking to the audience at one point, while Ariana looked on.

Justin Bieber
(Photo: The pair looked at ease and comfortable with each other on stage)

At times, the Biebs seemed mesmerized by Ariana's performance.

Justin Bieber
(Photo: Fans of the two stars have been hoping for a duet for over two years)

A little faux touchy-feely action between the duo.

Justin Bieber
(Photo: Justin Bieber and Ariana Grande getting cosy during one part of 'As Long As You Love Me' in Miami)

Justin posted a shared selfie with Ariana to his shots and Instagram accounts, post-show.

Justin Bieber
(Photo: The Biebs took a selfie of himself and Ariana backstage)

[Images via Entertainment Images/ Instagram/DC/Shots]