The European team booked their place in the last four on the back of a 3-2 win over Italy in the quarter-final and they were looking to Swiatek for another dominant start as the world No 1 had won all five her matches so far in the tournament - including two doubles encounters.
However, the three-time Grand Slam winner was brushed aside by world No 3 Jessica Pegula in the first match of the day at the mixed event as the American won 6-2, 6-2 in just over an hour, breaking three times in the first set and twice in the second.
Swiatek, who had won her four previous meetings against Pegula, was overcome by emotion after the defeat, but she was comfortable discussing the issue later on.
"Because I just knew that I felt kind of helpless today, because physically and mentally I wasn't able to kind of show up even, and problem-solve. So it's always hard when you lose, especially when you're kind of playing for the team and your country," the 21-year-old said.
She added: "So I was just sad. But, you know, it's not the first time I cried after a lost match. Nothing special."
Pegula's win gave her compatriot Frances Tiafoe some much needed inspiration as he went on to beat Kacper Å»uk 6-3, 6-3 to give the United States a commanding lead in the joint ATP-WTA tournament.
"When I saw her play today I was like, I can't come out here bulls***ing," he said during his on-court interview in Sydney.
"That was unbelievable, to beat Iga, who is obviously the best player in the world, that heavily. I think Jess is an unbelievable player, unbelievable person."
On Saturday, Hubert Hurkacz will look to keep Poland's hopes of a final alive when he takes on Taylor Fritz before Magda Linette goes up against Madison Keys in another must-win match.
Should the scores be tied 2-2, then the doubles will see Swiatek and Hurkacz face off against Pegula and Fritz in the mixed doubles.
The winner of the Poland-United States tie will face either Greece or Italy in the final in Sydney on Sunday.