Iga Swiatek left Spain reeling on Monday as Poland booked a 2-1 win to move into the quarter-finals of the United Cup whilst holders United States were beaten by hosts Australia.
The 2-1 victory for Australia propelled them into the last eight as well.
World number one Swiatek showed no mercy to Sara Sorribes Tormo in a 6-2, 6-1 singles victory before teaming up with Hubert Hurkacz to clinch the tie with a 6-0, 6-0 mixed doubles thrashing in just 53 minutes.
Hurkacz was ambushed in the day's opening singles, losing 3-6, 6-3, 6-4 to Alejandro Davidovich Fokina.
The top-seeded Poles finished Group A play with a perfect 2-0 record and await a last-eight opponent.
Swiatek, who has lost only seven games in two singles matches to start her countdown to the Australian Open, said playing mixed doubles was good experience.
"It's lessons for the future and great practice," the 22-year-old said. "I hardly ever play mixed. You use different skills, it's about feeling the geometry of the court."
Hurkacz credited his teammate with doing most of the hard yards.
"She carried me throughout the whole match, in every game she was making amazing shots," the world number nine said.
'Playing for our lives'
Australia had to work hard to join Poland in the quarter-finals but emerged with an excellent over the United States.
Alex de Minaur got the ball rolling for the hosts, nullifying the big serve of American Taylor Fritz 6-4, 6-2.
The 12th-ranked winner needed four match points and 90 minutes to win.
"Today was a little bit of "new year, new me," said de Minaur.
"I was in a great headspace, things worked out well.
"I played the type of tennis I wanted to play, I'm happy with that.
"He has explosive firepower and I could not let him dictate play."
Ajla Tomljanovic was unable to get past world number five Jessica Pegula, with the American levelling the tie after a tight 7-6 (8/6), 6-3 victory.
But Australia completed the evening's work in mixed doubles, with Storm Hunter and Matt Ebden defeating Pegula and Rajeev Ram 6-3, 6-1.
"We were playing for our lives," Ebden said.
"We knew we had to to everything possible, every point, every game."
Hunter said it was a very special moment.
"Playing in front of the home crowd and playing with Matty, a Perth local, it is a dream come true to win and go into the quarter-finals at home," she said.
In Sydney, France prevailed by the same score over Germany.
Germany's two-time ATP Finals champion Alexander Zverev needed to dig deep to get past Adrian Mannarino 4-6, 6-4, 6-3 and give Germany a 1-0 lead over France.
It seemed they would wrap up the tie when former world number one Angelique Kerber took the first set against Caroline Garcia 6-1, but she crumbled to lose the next two 6-2, 6-2 and send it to a mixed doubles decider.
Kerber, on her way back from maternity leave, returned to team up with Zverev against Garcia and Edouard Roger-Vasselin, with the French clinching the tie by winning the mixed doubles rubber 7-6 (7/4), 2-6, 12-10.
Earlier in Sydney, a sharp Casper Ruud won his second straight singles match before backing up in the mixed doubles to steer Norway past Croatia.
World number 24 Donna Vekic ground past unheralded Malene Helgo 7-5, 3-6, 6-3 to give the Croats a winning start to the Group F tie.
But three-time Grand Slam finalist Ruud swept past Borna Coric 6-4, 6-1 in just 89 minutes to level proceedings, then partnered with Ulrikke Eikeri to beat Vekic and Ivan Dodig 6-2, 3-6, 10-7.
It was a much-needed win for Norway, who lost to the Netherlands at the weekend.
"I'm motivated for the new year, I'm fresh and I'm ready," said world number 11 Ruud. "Another great singles match. I'm very happy to start the new year that way."
