Eric Clapton walks alone into the light and sits down. Wearing a blue shirt and khaki pants, cradling an acoustic guitar, he starts picking Big Bill Broonzy's "Key to the Highway" for a rapt crowd of 6,800 at the Forum in Copenhagen. The rhythm of Clapton's left foot, thumping the floor, is strong and clear. So is his voice. Two days short of his fifty-sixth birthday, Clapton sings Broonzy's travelin' blues, cut in 1941, with the leathery authority of one who has been on the road almost as long.

The next two hours are a soulful roll with his five-piece band through Clapton's own life story: "Tears in Heaven," "Bell Bottom Blues" and "Layla"; the autumnal pop of his new album, Reptile, dedicated to his late uncle, Adrian Clapton. Then at the end, after a gleaming "Sunshine of Your Love," Clapton sits down again with the acoustic guitar and plays an unexpected blues: "Somewhere Over the Rainbow," from The Wizard of Oz. Swinging slightly behind the beat, Clapton digs into the song with juke-joint force. But there is an exhausted melancholy in his performance too, the sound of a man nearing the end of his highway.

"It was born on holiday," Clapton says the next day in his kingly hotel suite, still wearing those khaki slacks, this time with a 1970s-vintage Stevie Wonder T-shirt. On a recent trip to the Caribbean, Clapton — who never travels anywhere without a guitar — learned "Somewhere Over the Rainbow," purely "to test my musical intelligence." He kept fiddling with the tune in tour rehearsals: "The sound guys were going, 'What the fuck is this all about?' " But the song, Clapton says, "insisted itself into the show." As a youngster in Ripley, England, a village in the Surrey countryside outside London, Clapton went to the cinema with his uncle, usually on Adrian's dates with his future wife, Eric's Aunt Sylvia. "I was their chaperon," Eric recalls. "We would see movies like Gunfight at the O.K. Corral, High Society and Carousel. The music from that period was so powerful.

"As I get into my old age," he continues, "those songs are coming up. I may have to do another couple of albums to get these things out of my system — before I can lay down and rest." Clapton delivers that line with a laugh. But he means it. Clapton's current world tour, he says, is his last.

Now in his fourth decade of recording, Clapton enjoys a unique, continuing relevance. He is virtually alone among his classic-rock peers in terms of his work rate — Reptile is his fourth studio project in five years — and sales. Last year's Riding With the King, Clapton's record with longtime idol B.B. King, sold more than a million copies. Personally, Clapton declares, "I'm in a good space with my domestic life." He is expecting a child by his girlfriend of the last two years, Melia McEnery, a twenty-five-year-old graphic artist from Ohio.

Yet Clapton admits that as he was planning this tour, which began in England in February and ends on August 18th in Los Angeles, "I was musing that it might be the last time. Now I'm going, 'This is definitely the last time.' It's hard. It doesn't work for me anymore. I get indigestion. I get tired." He grins. "Just talking to you — I'll pay for this.

"I will leave the door open for a couple of projects, to play the odd theater," says the guitarist, who has two albums left in his Warner Bros. deal. "But I'd say this was near the end.

"Anyone I talk to about it goes, 'Oh, you'll never stop.' I won't, in truth. I will always want to express something. But," he insists, "I don't need to do it like this anymore."

It's been ten years I've been doing this with Eric," says Clapton's rhythm guitarist, Andy Fairweather-Low, over coffee before showtime. "And I still haven't figured out why when he goes" — he makes a screaming-guitar sound — "it has that effect on you.

"I have one bit tonight that I do — I'll mull over what I'm going to do when it's my turn," says Fairweather-Low, a buoyant Welshman who sang with the 1960s group Amen Corner and made several fine solo LPs in the 1970s. "But Eric doesn't even think. Jump on him at any time, say 'Go!' and he'll take you to another level. Then if you say 'Once more,' he'll take you even higher.

"Jump on me," Fairweather-Low adds, "I'll freeze."

At the Forum with Fairweather-Low, bassist Nathan East, drummer Steve Gadd, keysman David Sancious and percussionist Paulinho Da Costa, Clapton plays with compact fire. In the 1970s, he could roll out diamond riffs for a quarter hour in a single blues like T-Bone Walker's "Stormy Monday." But tonight's high points include the precise shiver of Clapton's break in "River of Tears," from the 1998 album Pilgrim, and his gabba-gabba burst of wah-wah in the Reptile cover of J.J. Cale's "Travelin' Light." "I can't play long solos anymore without boring myself," Clapton contends. "I think it's important to say something powerful and keep it economical. That wasn't available to me as a younger player. I was motivated by ego. If I thought I was doing something good, I would do it all night."

Backstage, Clapton is a man without any visible superstar pretensions. In the band lounge, he greets visitors warmly, fixes his own cappuccino and tries to coax band mates and crew members into table-soccer matches. (It's a tough sell; Clapton is an ace player.) The owlish spectacles he wears offstage, combined with his close-cropped hair and grayish beard, lend him a professorial air. But it suits his encyclopedic passion for music. During the opening set by guitarist Doyle Bramhall II and his band, Smokestack, Clapton cocks an ear at a quote in one of Bramhall's solos. "'Stone Free,'" Clapton says quickly, referring to the Jimi Hendrix song.

Clapton is "definitely a fan of music," says the Texas-born Bramhall, who contributed songs and guitar to Reptile and Riding With the King. "I grew up playing the blues. I know some obscure artists. I thought I would hip him to some: Louisiana stuff like Lightnin' Slim." Bramhall laughs at his audacity. Clapton "knew all about them: the artists, the other people who played on the records, the producers."