The former Oasis star has been taking some time off after suffering with a chest infection, but he appeared to be in good health as he startled frontman Brandon Flowers with his appearance at Lollapalooza Festival.
Brandon can be heard in a video shared online, saying: ''F**king Liam Gallagher!''
The 'Run For Cover' hitmaker then proceeded with a cover of Oasis' 'Gas Panic!'
The track from 2000's 'Standing On The Shoulders of Giants' hasn't been performed by Liam and his bandmates - including his older brother Noel Gallagher - since their 2002 gig in Stockholm.
It's no surprise Liam chose the 'Mr. Brightside' hitmakers' set to stage invade, as the US band recently covered the Britpop group's hit song 'Wonderwall' at the Chile date of Lollapalooza.
Hours later at the music extravaganza, 'Wall of Glass' hitmaker Liam was forced to cut his set short because of the symptoms of his chest infection, which made it difficult for him to sing properly.
Liam told the crowd before leaving: ''My voice is absolutely smashed. ''I'm not just going to stand here all night screaming out for everybody sounding f*****g horrific.
''I'm very sorry I can't sing the tunes.''
The 'Paper Crown' singer was extremely ''disappointed'' that he had to walk away from the gig, but he was having problems with his breathing and felt it was best to stop his performance to avoid making it any worse.
Liam - who is set to return to the studio next month to work on the follow-up to his chart-topping debut solo LP 'As You Were' - vowed to return to Santiago in Chile at a later date.
Liam's representative said: ''Due to an ongoing chest infection, we had to cut the Lollapalooza Santiago show tonight short.
''Liam was diagnosed with a sinus, ear and nose infection after a European show a few weeks ago which has led to a chest infection.
''He has been continuing to perform and trying to push through it but unfortunately today he was finding it hard breathing on stage and wasn't able to continue the set.
''Liam is severely disappointed and upset that he couldn't finish the show and wants to apologise to everyone who came out to see him tonight.
''He promises to reschedule a gig in Santiago as soon as possible.''
Liam has been suffering with a chest infection since his concert in Paris on March 2.
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