LOS ANGELES-Britney Spears had no idea how big '...Baby One More Time' would be.

The 36-year-old singer kickstarted her career and a new era for pop music when she released the track almost 20 years ago, and though she recognised Max Martin had written a ''great'' track at the time, she didn't expect it to be so huge.

She said: ''The whole song is about that stress that we all go through as teens.

''I knew it was a great song. It was different and I loved it, [but] I don't think you can anticipate how a song is going to be received.''

And the 'Toxic' hitmaker can't believe it's almost the song's 20th anniversary.

She told the Guardian newspaper: ''Wow, that went quick. It was such a fun and crazy time, it was a bit of a blur.''

Britney hailed songwriter Max a ''genius'' and loves working with him because she respects his perfectionist attitude.

She said: ''I really respect that when I'm working with him. I think Max is a genius. It all just came together and felt right.

''In my opinion Max is the greatest songwriter of all time.''

To record the track, Britney flew out to Max's own Cheiron studios in Stockholm, Sweden, and though she was ''in awe'' of the city, she didn't get much time to explore.

She recalled: ''I remember being so in awe of Stockholm. I was out there for, like, 10 days, but we were so busy in the studio I didn't have time to go out and explore on that first trip.''

The song's co-producer Rami Yacoub remembered Britney was ''very shy and super sweet'' and had no idea how she'd become a pop megastar.

He said: ''I mean, she was a kid, and we had no idea there was a beast of an artist lurking under that innocent look.''