The 36-year-old singer has been singing with the 'Bohemian Rhapsody' group on tour since 2012 after impressing guitarist Brian May and drummer Roger Taylor when he performed a medley with the pair and the other finalists on 'American Idol' in the early part of that year and although he has been accepted by Queen fans he had major concerns about filling the late Freddie's shoes.
Speaking to PEOPLE, Adam said: ''When we first got together to perform a whole show, I'm happy to say it went really well. It was a success, and we pulled it off, but man, I was really nervous beforehand.
''Not only did I have to make sure my voice held up and I know all the words ... But it was also the perception of the fans; I definitely had some concerns.''
Even though he has been with Queen for six years, Adam still only considers himself to be a custodian of Freddie's tracks and he can never ever match what the rock icon achieved in his life.
And the pop star insists he is content to just keep alive the legacy of Freddie - who tragically died in 1991 aged 45 due to an AIDS-related illness.
He said: ''This is not my music that I wrote: This is me, as a guest, with one of the greatest rock bands of all time, singing the songs that one of the greatest rock singers of all time brought to life.
''Freddie is irreplaceable. There's no way to compare to him. It was always my hope that audiences would understand that I'm up there just excited to sing great music that everyone knows.
''And also to facilitate an opportunity for Brian and Roger, to keep being on stage and playing. Watching them delight in this connection with their fan base is really, really inspiring. This is what they were made to do; this is their legacy. And I'm just happy to be a part of it.''
Queen + Adam Lambert are currently playing a residency at the Park Theatre at Park MGM in Las Vegas throughout September.
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