Atomic Kitten star Natasha got together with Ritchie following The Big Reunion, which saw their bands and others reform for a TV show and tour.
Speaking about their relationship recently, Natasha described Ritchie as "very cute", adding: "We've been hanging out together a lot."
Last night, Natasha was in the audience rather than on the stage as she watched Ritchie and his bandmates - Scott Robinson, Abz Love and Sean Conlon - perform to hundreds of screaming fans in London.
Sharing a picture of 5ive on stage with her Twitter followers, Natasha gushed: "Absolutely awesome night @official5ive concert, my baby @RitchieNeville did us all proud:) Hammersmith Apollo rocked."
Before the show, Ritchie told his fans: "Londinium, the big smoke, we have arrived and we've got our party pants on #London #Hammersmith #FiveGHTour."
Although Natasha and Ritchie had know each other for years, they only grew close in September.
Speaking to Reveal in October, Natasha - split with her husband Riad Erraji in July after six years together - said that she and Ritchie were taking things slowly.
"At the end of the day, I don't know what's going to happen, but I do know that we get on very well!" she said.
"You know what, I've been through a lot this year and it's been great to have a friend who understands everything. We'll see what happens. It's hard to hold down a relationship with everything that's going on."
The Big Reunion stars have recorded a cover of 'I Wish It Could Be Christmas Everyday'. Watch the video here:
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