Murray, seeded No. 3, became the first men's semifinalist by beating No. 11 Nicolas Almagro of Spain, 6-4, 6-1.
Murray hit 15 aces to one for Almagro and never faced a break point. He had 29 winners and just seven unforced errors.
The Scotsman was runner-up to Roger Federer [ Images ] at Wimbledon last month. His opponent Friday will be the winner of the quarterfinal between No. 2 Novak Djokovic [ Images ] and No. 5 Jo-Wilfried Tsonga.
Kerber eliminated Venus Williams [ Images ] yesterday, but Azarenka feasted on the left-hander's serve, winning more than half of those points and breaking six times.
Azarenka, who won her first Grand Slam title at the Australian Open [ Images ] in January, seeks her first Olympic medal.
The winner of the event will be a first-time women's singles gold medallist.
Neither team managed a service break, and the Bryans sealed the victory on their fourth match point when Erlich double-faulted into the net.
The twins, who are top-seeded, won the bronze in 2008. Both are also entered in mixed doubles. The United States was the only country to still have an entry in all five singles and doubles events.