"My only number one hit in the world, and I forget the words," confessed an embarrassed Johnny Mathis, after drawing a blank in the middle of "Too Much, Too Little, Too Late," one of a few cozy duets he shared with Dionne Warwick at Wolf Trap last night. "This one, I know," he joked moments later, introducing "Misty." Needless to say, he crooned and the audience swooned.

The rare double bill not only drew a packed house but also quickly proved to be an inspired pairing of two of the more durable and distinctive voices in pop music. Among other things, Warwick performed a 30-minute, 23-song medley aimed at pleasing her fans who want to hear all of her hits, and even in abbreviated form, many of the old songs outshone the newer material. With help from a versatile orchestra and three background vocalists (all of them relatives), she breezed through a string of sophisticated and often melodically tricky songs by Burt Bacharach and Hal David and later delivered a beautifully subdued version of "You'd Be So Nice to Come Home To" from her recent album of Cole Porter tunes.

Not having Warwick's extensive parade of hits, Mathis could afford to lavish more time on his best-known recordings. His ethereal tenor voice, capped by a vibrato strong enough to set not just hearts but lawn chairs aquiver, never sounded better than when reprising "It's Not for Me to Say" or warming up to a few insinuating Brazilian melodies.

A repeat performance is set for tonight.