The singer tweeted good luck to his former bandmate on the eve of him kicking off three nights at the stadium.
"Have a great show tonight @robbiewilliams #O2london," wrote Barlow.
Williams played the first night of his three live shows at The O2 Arena yesterday (Thursday, November 22).
The 'Candy' singer has reunited with his former songwriting partner Guy Chambers for the trio of shows.
The event is in support of Williams's ninth studio album Take the Crown, which debuted earlier this month.
Watch the video for Candy below: