Golubic hit back to beat former U.S. Open champion Bianca Andreescu 2-6 6-3 6-4 before Bencic beat Leylah Fernandez 6-0 7-5 to give last year's runners-up an unassailable lead.
Friday's clash between the Swiss and the Canadians was effectively a group decider after both teams beat Italy.
Golubic replaced Jil Teichmann and initially the switch appeared to be backfiring for the Swiss as Andreescu ran away with the opening set.
But errors began creeping into the Andreescu game and Golubic raised her level to take full advantage, taking the second set and moving 5-2 in front in the decider.
Andreescu did manage to recover one break of serve after a medical timeout but Golubic sealed victory.
Bencic then came out all guns blazing against Fernandez to race through the opening set, but the Canadian dug deep in an absorbing second set before Bencic wrapped up victory.
Vondrousova only returned to competition last month after a lengthy absence due to injury but showed no fear as she beat the heavy favourite and Australian Open finalist Danielle Collins 6-3 6-3.
Collins converted on a chance in the fourth game of the first set but Vondrousova wrested back the momentum almost immediately, winning the next eight straight games.
She closed out the match winning slightly more than 80% of her first-serve points to secure the upset.
Siniakova appeared evenly matched with Roland Garros finalist Coco Gauff through a tense first set but costly mistakes derailed the American teen in the tiebreak.
Gauff never quite found her form in the second set, putting up 19 unforced errors as a near-flawless Siniakova never faced a break and roared with triumph after closing the affair 7-6(1) 6-1 for the 2018 champions.
