The hopefuls headed to the homes of regular judges Simon Cowell, Paula Abdul, Nicole Scherzinger and LA Reid. Focus there was on perfecting their vocals, and were stunned when the N.E.R.D star and the Hero singer rocked up to help them.
Previously announced guest judges Rihanna and Mariah Carey were also on hand to help the contestants before they face another round of elimination.
'The acts were totally blown away meeting their mentors at these amazing homes – then it was even more nerve-racking performing for a guest judge,' a source told People magazine. 'Some of the girls especially were delighted when they saw Enrique and Pharrell. The acts who met Rihanna were totally speechless – who knows how they managed to sing!
'Enrique has sold over 100 million records and there's nothing Pharrell doesn't know how to do in music. He's a solo artist and he's in a group, a writer, producer. That's massive pressure on these acts.
'Pharrell was a tough guest. He knew exactly the uniqueness he was looking for.'