  For extremely confined fluids with two-dimensional density $n$ in slit
geometry of accessible width $L$, we prove that in the limit $L\to 0$ the
lateral and transversal degrees of freedom decouple, and the latter become
ideal-gas-like. For small wall separation the transverse degrees of freedom can
be integrated out and renormalize the interaction potential. We identify $n L^2
$ as hidden smallness parameter of the confinement problem and evaluate the
effective two-body potential analytically, which allows calculating the leading
correction to the free energy exactly. Explicitly, we map a fluid of hard
spheres in extreme confinement onto a 2d-fluid of disks with an effective
hard-core diameter and a soft boundary layer. Two-dimensional phase transitions
are robust and the transition point experiences a shift ${\cal O}(n L^2)$.
