Watch: Taylor Swift and Avril Lavigne perform 'Complicated' together live.

Avril Lavigne is the latest celebrity to join Taylor Swift’s 1989 World Tour as a special-guest, surprising fans with an appearance at tonight’s show in San Diego to perform a duet of 'Complicated' together live.

"She’s one of my favourite artists and we’ve been texting all week. Today I got to meet her. Please make some noise! Avril Lavigne is here!"

Watch some clips of the performance below...

So incredible sharing the stage with @AvrilLavigne! 'Complicated' is such a brilliant pop song and I just love her. pic.twitter.com/JnE5ShK8ok

— Taylor Swift (@taylorswift13) August 30, 2015