The band shared the news on Twitter.com that they would play the NME/BBC Radio 1 Stage at 1.40pm, which came as a surprise to ticket-holders who had no idea the rockers would be performing.
In an intense six-song set, frontman Yannis Philippakis thanked the huge crowd crammed into the tent for coming to see them, and added that the festival "feels like our home".
In the vacant slot between Twin Peaks and Slaves, the Brit rockers kicked off their secret gig with My Number, before launching into Mountain At My Gates, Inhaler, Spanish Sahara, Red Socks Pugie, and finishing the set with What Went Down from their new album.
Festival organiser Melvin Benn dropped a hint earlier this week (beg24Aug15) as to who this year's surprise guests would be, and said, "Wild horses wouldn't let me be able to tell you who the secret guests are. You can take from that whatever you want."