Just hours before Metallica's concert at the Singapore Indoor Stadium, the band's lead singer and guitarist James Hetfield, 53, saunters into the interview room backstage with a clear cup of green juice.

It smells of kale.

It is the furthest thing you expect a rock star like him - the heavy metal guitarist and Metallica co-founder has been playing in the band since it was formed 35 years ago, through the height of sex, drugs and rock 'n' roll in the 1980s - to drink.

He points to the green juice as the band's secret to longevity. "Green sauce, singer juice," he says as he proceeds to sip it. "Mmm spicy, I think it has ginger in it."

Singapore is the band's last stop on a five-city Asian tour for their latest record, Hardwired...To Self Destruct, released in November last year.

"We're 35 years into this touring thing, so we're pretty good at it," he says, with an easy swagger that such a long career affords.

Lead guitarist Kirk Hammett, bassist Rob Trujillo and the band's co-founder and drummer, Lars Ulrich, make up the rest of the line- up of the Californian quartet.

He is well aware that they will be spending the next few years touring for the album, but he is unaffected. "It's a blur, my life's been a blur anyway," he says, nonchalantly.

Thirty-five years is a figure that keeps popping up during the interview.

Most bands who have been around that long are more likely to be on their farewell tours or milking a greatest hits setlist, not releasing new music.

Metallica, however, remain at the forefront, straddling both the niche heavy metal and commercial rock music scenes, and releasing new music consistently.

"First of all, I didn't think we'd still be alive and still playing in a band after 35 years," he jokes.

Their 10th album has been lauded by music critics the world over as a return to form for the band, whose early albums included the seminal best-selling Black album from 1991.

Recent releases such as Death Magnetic (2008) and St Anger (2003) were not as well received by critics and fans. St Anger was criticised for being too experimental while Death Magnetic never quite achieved the immortal status of the Black album.

Even Hetfield is taken aback.

"I'm so surprised that this album is getting such a positive response and people are needing it," he admits.

"It feels good to make a record that people are embracing as a new, viable album that they will have in their musical quiver."

They are no strangers to ups and downs, substance abuse and stints in rehabilitation.

Hetfield went into rehabilitation in 2001 for alcohol addiction while Ulrich famously had a cocaine addiction until 2008.

It seems, however, that their mojo is back and in a big way.

On Feb 12, Metallica will be performing live at the Grammy Awards where they are up for Best Rock Song for Hardwired.

"Music comes in trends and what not, but we haven't gone anywhere. If we're still popular enough to play the Grammys, that's cool," he says.

How they got their mojo back is not something he wants to ponder.

"I don't want to figure it out. What we have to offer is honesty and we love what we do. We're writing songs we want to hear and that's important," he says.

"We're not trying to recreate something that worked before or try a different genre or do some other crazy thing that's not us."

But even this veteran admits that "it gets harder and harder to surprise people ". Still, they try.

In a neat marketing trick, the band decided to release music videos for all 12 songs on their album. Typically, only a few singles in an album get music videos.

Three were released before the album dropped, but the remaining nine were each released on YouTube at hourly intervals, to coincide with the album launch.

Some were live-action videos and directed by famed directors such as Jonas Akerlund, but others were animations featuring up-and-coming artists and studios.

"The mystique and the surprise creates a different kind of excitement and energy, and I still search for that when we're releasing a new record," he says. "The joy now is that there is no one way to do it, there are multiple ways to get your music out - you just need to get creative with it."

The same sort of curiosity seems to have extended to his three children, whom he pulled out of school and brought out on the Asian leg of the tour for two weeks, but not before telling their school to "hand them a pile of homework" so they would not fall behind.

"There's a lot more to learn out here than sitting in school," says the globetrotting rocker, who says his children are exposed to music from all genres. They spent time in South Korea, China and Hong Kong on this tour.

His youngest daughter, 15-year- old Marcella, for instance, is into English classic rock act Led Zeppelin and American electropop duo Twenty One Pilots.

So what is in Metallica's future?

First, there is the remainder of the Metallica Worldwired Tour to get through, says Hetfield.

"We're pretty challenged with the new songs and wanting to play all 12 of them live at some point," he says.

He admits that perhaps waiting eight years to make the new album "was a mistake".

Will it be another eight years before the next album? "It won't be," he says reassuringly.