“Rosie was my first duet partner,” Mr. Bennett said, touching her face in the image. “I don’t remember what we sang. The CBS radio and TV show, ‘Songs for Sale,’ only gave us songs by amateur songwriters. They weren’t very good, but Rosie and I admired each other. When you feel that way, it shows in the singing.”

Sixty-eight years later, Mr. Bennett, 92, is once again singing songs built for two. On Sept. 14, he will release “Love Is Here to Stay” (Verve/Columbia), a duet album with jazz singer Diana Krall.

The album features 12 George Gershwin standards written with his brother and lyricist, Ira Gershwin, and with lyricists Buddy DeSylva and Ballard MacDonald. Mr. Bennett and Ms. Krall are accompanied by pianist Bill Charlap, bassist Peter Washington and drummer Kenny Washington.

“Diana didn’t want to know what I was going to do on each song,” Mr. Bennett said, seated on his sofa. “She’s very spontaneous, and I like that. I’d sing something and she’d come back with something else that complemented what I’d done. It was wonderful.”

Though Mr. Bennett and Ms. Krall toured together in 2000 and recorded individual song duets on his “Playin’ With My Friends” in 2001 and “Duets: An American Classic” in 2006, “Love Is Here to Stay” marks their first joint studio album.

Between 1950 and 1977, Mr. Bennett sang alone on most of his singles and albums. But on nearly all of his many TV variety-show appearances during this period and into the early 1980s, he was paired with dozens of classic pop, jazz and country giants including Louis Armstrong, Frank Sinatra, Ella Fitzgerald, Nat King Cole, Judy Garland, Patsy Cline and others.

“Singing a duet is very different than singing alone,” said Mr. Bennett, the winner of 18 Grammys. “You have to listen carefully to the other person. Your voices have to work together to give a song life. If you’re competing, it doesn’t work.”

When Mr. Bennett embarked on his celebrated comeback in 1986, he began recording duets with contemporary pop, soul and country singers, including k.d. lang, Elton John, Aretha Franklin, Amy Winehouse and Lady Gaga, among others.

“I love singing harmony on duets,” he said. “It’s total involvement. You have to anticipate where the other person is going and figure out instantly what to leave out and put in.”

Mr. Bennett’s new collaboration with Ms. Krall has a different feel than many of his recent duet recordings. Ms. Krall is a seasoned jazz singer and pianist, and a natural improviser who is at home on songbook standards.

In this regard, their collaborations on “Love Is Here to Stay” have a jazzy, free feel. There is also a contrasting marked difference between their vocal styles. Where Mr. Bennett’s voice has a sun-drenched, ascending optimism, Ms. Krall’s voice is nocturnal and breathy. The two styles nestle comfortably together and generate heat.

For instance, on “Do It Again” and “I’ve Got a Crush on You,” Mr. Bennett seems to dance across the melodies. By contrast, Ms. Krall’s husky voice curls seductively around notes, running counterpoint to Mr. Bennett’s trumpet-like delivery.

An accomplished jazz pianist who has accompanied Celine Dion, Barbra Streisand and Paul McCartney on albums, Ms. Krall ceded the keyboard on the new album to Mr. Charlap, widely considered to be one of today’s finest accompanists and a master of every jazz style.

The appeal of Mr. Bennett’s voice is rooted in the hope-filled approach of the street singer, an open vocal style that dates back to Renaissance Italy.

“A street singer in my old neighborhood in Queens was someone who wanted everyone to fall in love with the music,” he said. “I found it was the best way to communicate with audiences, starting with my family during the Depression.”

Ms. Krall sings alone on the ballad “But Not for Me” with a tender, conversational style accompanied only by Mr. Charlap. Mr. Bennett’s solo turn comes on the peppy “Who Cares.” On the line, “I love you and you love me and that’s how it will always be and nothing else could ever mean a thing,” Mr. Bennett sings the line using a spiraling, ascending arpeggio.

“I love improvising—it’s why I’m still singing today,” he said. “It’s difficult to describe. When I improvise with Diana or alone, I push myself to sing things I haven’t done before. I’m really into it. I like being in that mood.”

—Mr. Myers, the author of “Anatomy of a Song” (Grove), is a regular Wall Street Journal contributor.