Rap newcomer Iggy Azalea's surprise set with Jennifer Lopez fell flat in Chicago, Illinois on Saturday (14Jun14) after her microphone failed to work.

The Jenny from the Block superstar headlined the B96 Pepsi Summer Bash and thrilled fans when she brought out the Australian performer.

However, as Azalea was about to break into her rap on Lopez's track Acting Like That, technical difficulties shut off her mic.

The sound team at the event eventually fixed the problem, but Azalea was furious when she left the stage.

Taking to Twitter.com, she wrote, "I was supposed to sing actin (sic) like that with JLo at summer bash today but some idiot handed me a microphone that wasn't even turned on."

Despite the incident, Azalea admitted she felt honoured to perform with a superstar like Lopez.

She added, "Watching this woman do her thing tonight was beyond impressive. We had some technical difficulties when it was time for our song but hey, I still enjoyed the opportunity to stand on stage next to someone I've admired for a long time. jlo you're the most down to earth diva there is, always smiling & a true class act. wishing you so much luck for your album release!"