The X Factor judge performed a headline set at the Royal Albert Hall when he invited his Take That bandmate to join him on stage.
Williams and Barlow began by singing 'Eight Letters', before they duetted on the former's recent chart-topper 'Candy', which they co-wrote together.
However, Barlow's gig was nearly cancelled after the water was turned off throughout the whole building, according to The Sun.
"At the moment the show's off – there's no water in the building and because of health and safety you can't start a gig without it," Barlow is quoted by the tabloid as saying backstage.
"Hopefully it comes on soon because at the moment I'm not going to be playing tonight."
Eventually, the water supply returned and the show was given the go-ahead.
Barlow will continue his UK headline tour of the UK at Sheffield's City Hall tonight (November 28).