US rapper Eminem. Picture: Bang Showbiz

Eminem has opened up about how hard it was for him to return to the recording studio and rap after entering rehab for drug addiction in the early 00s.

The 45-year-old rapper - whose real name is Marshall Bruce Mathers III - has opened up about the quality of each of his albums, including 'Relapse' which was written at the end of his rehabilitation for addiction to prescription sleeping medication, and was recorded between the period of 2007 and 2009.

The 'Walk On Water' hitmaker said he was so "scatterbrained", he felt like he would have to learn how to rap again.

In a candid interview with Vulture magazine, he admitted: "'Encore' was mediocre.

"And with 'Relapse' -- it was the best I could do at that point in time. ['Relapse'] was a funny album for me because I was just starting back rapping after coming out of addiction. 

"I was so scatterbrained that the people around me thought that I might have given myself brain damage. 

"I was in this weird fog for months. Like, literally I wasn't making sense; it had been so long since I'd done vocals without a ton of Valium and Vicodin. I almost had to relearn how to rap."

The 'Lose Yourself' hitmaker is back with new LP 'Revival', and says whether people like his new music or not, he feels he's the "best at rhyming" he's ever been.

However, he admits that he felt there was a lot of pressure to beat the success of 2000's 'The Marshall Mathers LP', which featured the star's biggest hits such as 'Stan' featuring Dido, 'Real Slim Shady' and 'Kim'.

Asked if he still has a competitive streak in him, Marshall said: "I am forever chasing 'The Marshall Mathers LP'.

"That was the height of what I could do. I just don't have the rage I did back then. 

"If I did, the music would be the same, and I hope it's changed. 

"And if I still had that rage it would mean I wouldn't have grown as an artist or a human being. Technically I feel like I'm better at rhyming than I've ever been."