Well I don't know but why you are in doubt of under performing at JEE advanced. 

I would like notify you that JEE advanced and JEE mains are entirely different. I've personaly observed many top ranking aspirants having rank around 20k or even over 30k in mains getting a position under 5k rank i JEE advanced.

The reasons are that

1. They have prepared indepth suiting for JEE advanced

2. They might not have good grip over JEE mains specific topics like statistics, probability distribution or magnetism or optical instruments. This resulting in a loss of good number of marks and hence their rank got poorer.

3. A lot depends on luck. Yes not all hardworking aspirants reach the final destination. (though JEE advanced I believe is the paper which is one of the most fair competitive exam)

So cheer yourself and prepare as hard as you can for JEE advanced. Remember every hours at this stage counts and successful students are those who knows the importance of this hour. 

Thank You!