The pair appeared to be in Singapore roughly 18 hours before featuring at Auckland's WTA tournament.
Barrett posted a video on his Instagram story with Williams in the background, and the caption "watching greatness".
Photo / InstagramHannah also took to Instagram when watching Williams, saying "so special getting to watch this legend play".
Williams ended up victorious, beating Italian qualifier Camila Giorgi in straight sets.
Fellow All Blacks Damian McKenzie and Anton Lienert-Brown were also in attendance.
Auckland is Barrett's new home, having moved to the Blues from the Hurricanes as part of a new deal he penned with New Zealand Rugby in July.
He's set to suit up for the Super Rugby franchise in mid-April after a break from rugby, which is part of his new contract.
The Blues open their season against the Chiefs on January 31.