Matchings in hypercubes extend to long cycles

The $d$-dimensional hypercube graph $Q_d$ has as vertices all subsets of $\{1,\dots ,d\}$, and an edge between any two sets that differ in a single element. The Ruskey-Savage conjecture asserts that every matching of $Q_d$, $d\ge 2$, can be extended to a Hamilton cycle, i.e., to a cycle that visits every vertex exactly once. We prove that every matching of $Q_d$, $d\ge 2$, can be extended to a cycle that visits at least a $2\mathrm{/}3$-fraction of all vertices.