As a former champion, he has the right to do so.
4 seconds sound clip from the Mortal Kombat (1995) movie soundboard.
You can hear this line at 00:39:09 in the Blu-ray version of the movie.
Quote context
[...]
- You guys are lucky he stopped us.
- So now you've seen what you'll be facing in the tournament.
- Will Shang Tsung fight in the tournament?
- As a former champion, he has the right to do so.
- And he's far more dangerous than Goro.
- His power comes from the souls of vanquished warriors.
- To fight Shang Tsung is to face not one, but a legion of adversaries. Remember that.
- Tomorrow the tournament begins. Be prepared.
[...]