Use your hazard lights.
Drive out of the tunnel if you can, but don’t make a U-turn or reverse.
If you can't drive out, drive to a lay-by, an emergency lane or pull over as far to the nearside as you can.
Turn off your engine but leave the key in.
Use an emergency phone to call for help.
Don't open the bonnet, which may be hot and can increase the fire.
Don’t try to put the fire out yourself, unless it’s only just started.
If you can’t put the fire out, leave quickly using emergency exits.
Don't waste time picking up personal belongings.
Help injured people get to safety too.

It's important to remain calm and react quickly in such a situation! You can check these articles out if you need a guide!

1. What should you do if your car catches fire:
www.motorist.sg/article/178/what-should-you-do-if-your-car-catches-fire

2. Everything you need to know about tunnel driving in SG
https://www.motorist.sg/article/171/everything-you-need-to-know-about-tunnel-safety-in-singapore