Pop icons Lorde and Taylor Swift have grown close as both have been in the international spotlight, and last night (July 13), Lorde proved her friendship by taking a 19-hour flight to surprise fans at Swift's Washington D.C. concert with a duet of her international hit "Royals." Check out a clip of the performance below.

And of course, the two discussed the performance and their friendship on Twitter:

Swift's tour will continue through the rest of 2015, so check out her upcoming schedule at her website. We're guessing this isn't the last we've heard from the pair -- and we also suspect this won't be the last time Swift is "surprised" at a concert by a high-profile guest. Who's next?

As for Lorde, it's likely she's currently working on her next album, as she said she would be back in February. We'll keep you posted as soon as we hear anything -- though, we're sure the internet and alternative radio stations across the country will explode the second we get a taste of new music from Lorde.

In the meantime, enjoy Lorde and Swift's recent collaboration: