If Willie Nelson's ethereal brand of pop- country represents a break with Nashville tradition, Don Williams' gently rolling ballads represent a brilliant refinement of that formulaic structure. Williams has defined a particularly approachable, unpretentious style, so laid-back that he's been referred to as the Gary Cooper of American music. The quiet man of country, a maverick in his own right, has "quietly" been walking off with gold records and major awards since 1975, "quietly" becoming one of country's most unlikely superstars.

His latest album, "Listen to the Radio," contains ten songs cut from the same sturdy oak as earlier albums; and though Williams is a superb songwriter, he's only co-authored one tune this time, "If She Just Helps Me Get Over You." The rest, coming from such established tunesmiths as Bob McDill, Fred Knipe and Jennifer Kimball, have been absorbed and remade into highly personal and affecting statements.

When you talk about Williams, you have to start with his voice, one of the warmest and most distinctive in pop music. His unaffected baritone is as rich and languid as maple syrup or honey; it sticks to the roof of your mind. Because Williams chooses sobering, wistful portraits of love and loss, there's a low-key, structural unanimity to his work that throws some people off, as if mellowness were offensive. Songs like "If Hollywood Don't Need You," "Only Love," "Standin' in a Sea of Teardrops" and the title cut (currently a top country hit) chug gently along, while the singer ambles through the lyrics as if they were an afterthought. For the most part, the sympathetic backing is spare and lean; all the attention is on Williams' sensual delivery. The total effect is at once soothing and stimulating, wonderful music for slow dancing and fast thinking. They may come livelier than Williams, but they don't come much better.

Lee Greenwood, who opens for Williams in concert tonight, follows a similarly mellow regimen, occasionally unveiling a harder edge, as on the R&B-flavored "Ain't No Trick (It Takes Magic)." His voice has touches of Bill Medley, Gary Puckett and Bob Seger, warm but with slightly rougher edges. Greenwood's offerings follow the more traditional country breakdown: hurtin' songs ("Ring on Her Finger, Time on Her Hands," "Broken Pieces of My Heart," "She's Lying"); and healin' songs ("Love Don't Get No Better Than This", "Thank You For Changing My Life"). "Inside and Out" is Greenwood's first album, a promising start that complements Don Williams' charismatic quietude.

THE ALBUMS

DON WILLIAMS -- "Listyen to the Radio" (MCA 5306).

LEE GREENWOOD -- "Inside and Out" (MCA 5305).

THE CONCERT WILLIAMS AND GREENWOOD -- 8 p.m. Friday at Marshall High School Stadium, Falls Church (276- 8035).