How to draw a line on a symbolic surface

I would really like a line running along the surface below at all points where f1=0. kind of like a contour line in 3d.

syms x1 x2 
f1 = x1^2+x2^3-9

f1 = 

fsurf(f1);

Answer 1

[John D'Errico]

syms x1 x2 
f1 = x1^2+x2^3-9

f1 = 

fsurf(f1);
hold on
fimplicit(f1,'r')

WTP? :)

[Rowan Humphreys]

You are a gift from god himself. Thank you so much.