  We study the ordinary differential equation ${\varepsilon}\ddot x+\dot x +
{\varepsilon} g(x) = {\varepsilon} f(\omega t)$, with $f$ and $g$ analytic and
$f$ quasi-periodic in $t$ with frequency vector $\omega\in R^{d}$. We show that
if there exists $c_0\in R$ such that $g(c_0)$ equals the average of $f$ and the
first non-zero derivative of $g$ at $c_0$ is of odd order $n$, then, for
${\varepsilon}$ small enough and under very mild Diophantine conditions on
$\omega$, there exists a quasi-periodic solution close to $c_0$, with the same
frequency vector as $f$. In particular if $f$ is a trigonometric polynomial the
Diophantine condition on $\omega$ can be completely removed. This extends
results previously available in the literature for $n=1$. We also point out
that, if $n=1$ and the first derivative of $g$ at $c_0$ is positive, then the
quasi-periodic solution is locally unique and attractive.
