Australia defeated the United States 3-2 while Romania upset defending champion Czech Republic by the same score as both advanced to the semifinals of the Fed Cup on Sunday along with France and Belarus.
Earlier, Barty beat Madison Keys 6-4, 6-1 to give Australia a 2-1 lead. Collins, who reached her first Grand Slam semifinal at the Australian Open last month, made it 2-2 after a 6-1, 3-6, 6-2 victory over Daria Gavrilova.
Romania reached the semifinals for the first time after its first victory against the Czechs, who won six titles in the past eight years.
Romania took a 2-1 lead on an indoor hard-court in Ostrava after Simona Halep beat Karolina Pliskova 6-4, 5-7, 6-4 in a reverse singles featuring two former top-ranked players. Halep improved her record against Pliskova to 7-2 to put Romania one win from advancing.
Siniakova, the star of last year's final victory against the United States, made it 2-2 with a 6-4, 6-2 win over Mihaela Burzanescu, leaving the result hinging on the doubles.
The Australians will next play Belarus, which lost the 2017 final to the U.S., in April. Belarus dominated at Braunschweig, with Aryna Sabalenka cruising past Laura Siegemund 6-1, 6-1 to give her country an insurmountable 3-0 lead over host Germany on an indoor hard court.
The ninth-ranked Sabalenka hit 18 winners compared to five by her opponent to help Belarus avenge a 3-2 loss to Germany last year in Minsk.
Garcia marked her return to the Fed Cup for the first time since the 2016 final with a couple victories after overcoming Alison Van Uytvanck 7-6 (2), 4-6, 6-2 in the opening singles on Saturday.