Tomic had kept Australia alive in the contest after Stosur's winless record against Pennetta was stretched into a fifth match.
Pennetta won 6-4 6-4 before Tomic battled back from a set down to beat Seppi 4-6 6-3 6-2.
The Australian's efforts left him with little in reserve for the following mixed doubles as the hosts, who lost their Group A opener to Canada, saw their hopes of ending a 16-year title wait in the Western Australian capital ruled out.
"This week is a training week for me. I'm playing a little bit tired, but who cares," Tomic told the tournament's official website.
"At the end of the day, I've got to push myself.
"I know I was feeling a bit tired out there, but it's only going to make me stronger for the big one (the Australian Open) in a few weeks."
The second-seeded United States pairing of John Isner and Sloane Stephens meet French duo Jo-Wilfried Tsonga and Alize Cornet in the only tie on New Year's Day.