Katy Perry is on course to outsell Lady GaGa in their chart battle.

The 28-year-old pop star's new single 'Roar' - which is the first track lifted from her upcoming album 'Prism' - is expected to sell double the amount of Gaga's new single 'Applause' in the United States after they were both released on the same day last week.

According to Billboard, Katy will shift between 525,000 and 550,000 copies of her comeback song, marking her highest first week sales on the US singles chart, while Gaga is set to sell 200,000 to 225,000 copies of her song.

Both Katy and Gaga will use their new singles to launch their third studio albums. Katy's 'Prism' is set for release in October, while the 'Bad Romance' hitmaker will release 'ARTPOP' in November.

Gaga's modest first week sales are a far cry from the 448,000 downloads she sold in just three days in 2011 when she released 'Born This Way', the first single from her second album of the same name.

The 27-year-old star rush-released 'Applause' in the US last Monday (12.08.13) after a snippet of the track leaked online.

In the UK, the song debuted at number five on the singles chart yesterday (18.08.13).