Playing flawless tennis, Barty did not drop serve during the 6-3 6-4 victory before teaming up with Matthew Ebden to clinch a tight mixed doubles clash 3-4(3) 4-3 4-3(3) against Muguruza and David Ferrer for Australia's second straight win in the event.
Ferrer had capitalised on Ebden's 42 unforced errors to prevail 7-6(1) 7-5 and level the contest at 1-1 but despite his encouraging early form, the Spanish veteran said he would not reverse his decision to retire this year.
Earlier, last year's runners-up Germany defeated France 2-1 to top Group A with a second victory in as many ties, improving their chances of playing in Saturday's title clash.
Wimbledon champion Angelique Kerber fought back from a set down to defeat a determined Alize Cornet 5-7 6-2 6-4 for her second straight singles win following a victory over Spain's Muguruza in Sunday's opener.
Alexander Zverev then sealed the tie for Germany when he fought off Lucas Pouille 6-3 6-7(8) 6-2, before France claimed a consolation win in the dead mixed doubles rubber.
"You want to play tough opponents, you want to play the best players before a Grand Slam," 2018 ATP Finals winner Zverev, who will be one to watch in this month's Australian Open, said.
"I've played two good players, and I'm happy to get those matches under my belt. I hope to keep going with Angie, and we'll see what else we can do."
Germany can seal a place in the final with a win against Australia on Friday.
