Rak-Su beat Grace Davies in a the new sing-off twist for this year's live shows which pits the best acts against each other.
This year there is no bottom two sing off and the results are on the same night, decided by the viewer poll alone.
On Saturday the Boys and Groups performed and duo Sean and Conor and Sam Black became the two acts to be eliminated after polling the fewest votes.
But Rak-Su went forward to the 'prize fight' after topping the public vote for a third time.
Grace and Rak-Su therefore went head to head in the prize fight, competing to spend studio time in LA with a top songwriter.
The two both reprised their original songs from the weekend before another public vote opened between the two.
Once the flash poll closed, host Dermot O'Leary revealed the results.
And with the most votes, Rak-Su won the fourth prize fight of the series and will enjoy studio time in LA with a top songwriter.
A total of six now remain in the competition's final stages ahead of next weekend's Semi Final live shows.
They are Simon Cowell's Groups Rak-Su and The CutKelvins, Sharon Osbourne's Girls Grace Davies.
Plus Louis Walsh's Boys Lloyd Macey and Nicole Scherzinger's Overs Kevin Davy White and Matt Linnen.
The X Factor 2017 continues next Saturday and Sunday night on ITV.