The songwriter and musician is overseeing production of the singer's entire new record Glassheart and has described it as "relevant".
It's the first time Lewis and Smith have worked together on a complete album, with Smith producing the whole project and co-writing half of the tracks.
"It's been amazing," he told Music Week. "It's been a great challenge to come up with a vision at the beginning of how we wanted it to sound and I hope to be able to do more projects like that in the future.
"People have preconceptions of what Leona is so you want to keep the essence of that and keep pushing the boundaries.
"We've got the key tracks and we're very close. She's done some fantastic vocals and is making a very relevant record and a very important record for her. The songs are really strong."
As well as confirming that work on the album is about 80% finished, he asserted that Lewis is "making an artist album rather than it being a collection of songs".
Fraser T Smith is best known for co-writing Adele's 'Set Fire to the Rain' and Taio Cruz's 'Break Your Heart'.
Lewis recently promised fans that she will perform a new track from Glassheart - due out November 26 - at Radio 1's Hackney Weekend on Saturday (June 23).
Watch the music video for Leona Lewis's 'Collide' below: