Vandeweghe beat Lucie Hradecka 6-4, 6-2 of Czech Republic before Sock defeated Adam Pavlasek 7-5, 3-6, 6-3 to clinch the match ahead of the mixed doubles, which this year will feature the Fast4 format, including a tiebreaker at 3-3, no advantage and gender-to-gender serving.
Hradecka replaced two-time Wimbledon champion Petra Kvitova after Kvitova was injured 10 days ago in a knife attack by an intruder in her home in the town of Prostejov. Kvitova is expected to be off the tour for up to six months.
”I played a decently clean match, it’s always going to be a little bit interesting your first time out on the court,” Vandeweghe said. ”A little pre-match jitters but I thought I did a good job.”
The United States has won the Hopman Cup a record six times in the tournament’s 28-year history.
Defending champions Australia, represented by Russian-born Daria Gavrilova and Nick Kyrgios, were scheduled to play a late match Sunday against Spain.
Roger Federer, making his return after a six-month left knee injury layoff, and Belinda Bencic begin play for Switzerland against Britain, represented by Heather Wilson and Dan Evans, on Monday night.