Will performed alongside his finalist from this year's series as well as other members of the Black Eyed Peas.
Lydia made it through to the final back in April but was beaten to the win in the public vote by Kevin Simm.
— Joeann Murphy (@joeann_murphy) May 11, 2016
On the live show Will had promised fans a free gig if Lydia won and even though she didn't quite manage it he said the concert would still go ahead.
Speaking during The Voice UK's live final last month, Will vowed to viewers that he would work hard to help Lydia become a "superstar."
"I love the show, we have to make sure that someone's a superstar and has a song in the top 10..." he told hosts Marvin Humes and Emma Willis. "I'm trying my hardest to make that happen."
There was both an evening (5PM) and late night performance (9PM) yesterday evening at the Royal Albert Hall with completely free tickets having been quickly snapped up.
Will and Lydia reprised their duet Boys & Girls from The Voice's final as well as singing a mash up of Heatbreaker and Sorry together.
Plus, Lydia performed on Black Eyed Peas hits such as Where Is The Love and I've Gotta Feeling.
— Jade (@Jadey_Brady) May 11, 2016
Those in the audience were more than impressed, with one tweeting afterwards: "Genuinely one of the best concerts I've ever been too!"