Joe Williams, who opened Tuesday night at the Showboat in Silver Spring for six nights, has the unusual gift for singing popular tunes and blues melodies with equal and - consummate - ease.

So many vocalists have failed to master both the delivery of an earthy blues lyric and a more pristine Ira Gershwin line. But not Williams. He is deft as a master builder at conveying a highly personal sense of melodic and lyric development.

The other night, for example, he dug into his repertoire and pulled out "In the Evening," one of those pungent blues that a singer can make even more poignant by a simple wist of lyric or melody. Williams did that - and he went further by interpolating the self-mocking "Rocks in my Bed."

He went on to sing another blues, "Just a Dream," a celebration of the humor of illusion - (he dreams that all his eight children looked like him, but when he wakes up not one resembles him).

Previously, he had explored the more elegant contours of "There is No Greater Love" and "There Will Never Be Another You." On both songs he seat sang in a breezy, good-humored fashion.

Williams was superbly accompanied an opening night by pianist John Malachi, bassist Billy Taylor and drummer Bertell Knox. Malachi will be replaced for the rest of the week by Dick Morgan.