CHICAGO – By his own admission, Tony Levin, the accomplished bass player who has worked with John Lennon and Paul Simon, tours with Peter Gabriel and handled complicated musical arrangements alongside virtuoso guitarist Robert Fripp for nearly 40 years, is the laziest member of King Crimson. "Not an answer I'm proud of!" he says with a laugh, by phone from a hotel in Guadalajara, Mexico. "I practice plenty for a guy who's been playing as many years as I have. I joke that it ought to be more – but I forgive myself for being less than ideal in that."

Like the other seven members of King Crimson, the progressive-rock band Fripp started with 1969's mighty debut "In the Court of the Crimson King," Levin actually keeps a grueling rehearsal schedule. Every day on tour, Fripp, the 73-year-old bandleader, puts on a suit, eats breakfast, then starts practicing his instruments. By the time Levin, also 73, arrives at whatever concert venue they're playing that night, Fripp is in his dressing room, continuing his musical regimen.

Fripp, who doesn't do phone interviews, chooses the material for daily sound checks that are more like rehearsals, often tossing out unexpected choices from the band's 50-year career. None of it is easy – King Crimson is famous for unusual rhythmic timing, busy arrangements that touch on classical and jazz and complicated solos that are both choreographed and improvised. At sound check, the band runs through the new material repeatedly, then the members disperse to their dressing rooms to practice individually.

"In the bands I've toured with, this is the hardest working, by far – really, by a long shot," Levin says. "This is not music where somebody can go, '1-2-3-4, come in.' It's hard enough to play when everyone does it right, but if anybody makes a mistake, there's no set rule of how to recover. You just need to experience that a lot."

After playing with several British combos, Fripp created King Crimson in early 1969 and quickly took the band to its first performance, opening for the Rolling Stones with an estimated 650,000 fans in attendance. The band's first few albums, including 1970's "In the Wake of Poseidon" and 1973's "Larks' Tongues In Aspics," are progressive-rock touchstones, albums that showed how thrillingly complex the genre could be, influencing bands from Yes to Rush to Phish.

Fripp, also a master of a keyboard called the Mellotron, has over the years taken a dogmatic approach to King Crimson, refusing to play older material in concert and declining to make its catalog available on Spotify and other streaming services. (The band's manager has said CD sales were robust, until recently, so it didn't make sense to kill that business with streaming.) But in 2015, he returned to the older material in concert, and, earlier this year, began streaming all the classic albums; the band also posts numerous concerts, like "Meltdown (Live in Mexico, 2017)," at three hours and 39 minutes, full of old and new songs and, of course, plentiful solos.

Playing older material was "something new," Levin says. "We had either avoided it completely or done it in a minimal way. In this ensemble, all of us are ideally suited for that, because we can cover a lot of sonic material. Robert suggests, 'Hey, let's look at this piece from the '70s,' and I need to go through a process of determining what is so special about that bass part that I want to keep it, and where there's room for me to be myself. It's been really challenging."

Levin first learned the upright bass when he was 10, growing up in Brookline, Mass., a suburb of Boston. He asked his parents, recently, why he chose that instrument, and "they just said I liked it," he recalls. The bass evolved into his career, and over time he also mastered the Chapman Stick, a wider instrument that can play more notes between the notes. "It was a pretty profound decision I made when I was 10," he says. "You could call it a lucky decision, for sure."

After studying at Eastman School of Music and playing under Igor Stravinsky himself, Levin moved into rock, accompanying ex-Genesis singer Gabriel on his 1977 solo debut. The album's producer, Bob Ezrin, brought in Levin; Fripp played guitar on the same session. Over time, Levin has played with John Lennon, Pink Floyd, Lou Reed and James Taylor, becoming the go-to player for bandleaders who need confidence with difficult bass parts. None are as difficult as King Crimson, which he joined in 1981. Its three-drummer lineup forces Levin to switch knobs on his onstage sound monitors, making on-the-spot decisions on which player to lock in with.

In addition to working with King Crimson, Levin is in a Chapman Stick-focused band called the Stick Men, and he tours with his keyboard-playing brother, Pete, in the jazz combo Levin Brothers. All these side projects are mostly live _ there's no time to make recordings. "Because I'm not home much, those projects unfortunately need to wait, sometimes many years, before they come to fruition," Levin says. "The trick is to get time at home to actually enjoy them."