Seems like Stefanie Sun had spent her time in isolation very productively.

In the wee hours of Friday, the Singaporean singer, 42, released a surprise single, What Remains, an introspective tune she wrote, composed and produced during the pandemic.

The last time she did so was 19 years ago, for the song Someone, which is on her 2002 album Start.

The new single, which has been released on various digital platforms, features her signature lilting vocals, tinkling piano accompaniment and reflections on isolation.

"I didn't deliberately make this song sad," she said in a press statement. "What I want to bring out is a feeling of being overwhelmed with emotions."

She was supposed to have been holding concerts around the region last year, as it was the 20th anniversary of her debut, but was instead stuck at home.

The downtime gave her the chance to reconnect with her loved ones, reflect on the past and pen a heartfelt song.

A key line in it goes: "I haven't said how much I've missed you for fear that I'll miss you once I say it."

She added in her press release: "I hope those who listen to it can rediscover themselves, tidy up their emotions and then move on from it."

The best-selling Mandopop artiste, who is married with two children, is known to spring surprises on her fans.

Last June, she held an unexpected online concert to mark the 20th anniversary of her debut hit album, Yan Zi.

That was followed by another show in October, a live-stream of an intimate gig at Pasir Panjang Power Station which was watched by 15 million fans online.