Demi Lovato is so proud of herself for staying sober for five years but admits she has had her fair share of ups and downs and times when she wanted to relapse. PICTURE: Instagram

Demi Lovato is to perform the US national anthem ahead of Floyd Mayweather's epic boxing bout against UFC star Conor McGregor.

The 25-year-old star will take to the stage at the T-Mobile Arena in Las Vegas on Saturday to belt out the iconic 'Star-Spangled Banner' tune ahead of one of sport's most hotly-anticipated events of all time between the professional boxer and the UFC fighter. 

Justin Bieber 'distances himself from Floyd Mayweather'

A tweet on the account of Mayweather Promotions - of which Floyd is president - read: "[email protected] set to sing the National Anthem prior to Floyd & Conor making their way to the stage on fight night! (sic)"

TV channel Showtime also tweeted: "Excited to have #Grammy nominated and multi-platinum global artist @ddlovato sing the National Anthem at #MayweatherMcGregor! (sic)"

Demi confirmed the news by retweeting both of the messages on her own Twitter account, and it won't be the first time she has performed the US national anthem. 

The raven-haired star sang the famous tune at game four of Major League Baseball's 2015 World Series, game five of the 2011 World Series and at an American football game involving the Dallas Cowboys in 2008.

But it will be the first time she has performed the song at a boxing match.

Demi will be joined by a host of stars at the so-called "Fight of the Century", including Angelina Jolie, Charlize Theron, Mark Wahlberg, Drake, Denzel Washington and L.L. Cool J.

Some celebs are expected to arrive in Vegas earlier this week, and that certainly seems to be Demi's plan. 

She tweeted: "See you on Thursday in Vegas?? We're shooting something special with @MTV (sic)"

Jolie, Theron to attend Mayweather-McGregor fight

Actor Jamie Foxx, basketball sensation LeBron James and Maroon 5 frontman Adam Levine are also expected to attend the fight. 

The bout is particularly eagerly anticipated because it will see undefeated boxer Mayweather taken on an Ultimate Fighting Championship (UFC) star in McGregor, in what will be the Irishman's first fight as a professional boxer.