Swiatek helps Poland sweep past Brazil in United Cup
Swiatek led her top-seeded nation to the opening group win, crediting a calming few weeks away from the game after lifting the year-end women's championships last month in Mexico.
"It was peaceful for the first time, I was able to focus on practice and rest," Swiatek said after dominating Beatriz Haddad Maia 6-2, 6-2.
"I hope to be more dangerous this season - I'll do my best," she said.
"I'm having such fun here and can concentrate on tennis," added the 22-year-old three-time French Open champion.
Swiatek said the victory over Haddad Maia showed off her recharged mental strength.
"I worked hard in the pre-season and gained a lot of confidence. I was able to play freely without any other thoughts to mess up my game."
Teammate Hubert Hurkacz secured the day's second win with a 6-7 (4/7), 6-2, 6-3 comeback over Thiago Seyboth Wild to put the tie out of reach.
'Just a normal match'
Earlier, Zheng served up an upset 6-1, 2-6, 6-1 win over the seventh-ranked Wimbledon titleholder Vondrousova.
"It was just a normal match for me," said Zheng, the WTA's 2023 Most Improved Player who has risen to be world number 15.
"It doesn't matter who I face. I'm happy with the win and excited for my team," the 21-year-old added.
It followed teammate Zhang Zhizhen defeating Jiri Lehecka 5-7, 6-3, 6-4 in China's maiden appearance at the mixed teams tournament.
Zhang held his nerve against Lehecka in a heavy-hitting clash, with the world number 58 dictating from the baseline.
Alexander Zverev battled back from a set down to beat Italy's Lorenzo Sonego 6-7 (5/7), 6-3, 6-4 and earn a point for Germany in Sydney to get his season off the a winning start.
Jasmine Paolini had given Italy a 1-0 lead, surviving a bout of cramps to defeat Angelique Kerber 6-4, 7-5 in the first match of the German former world number one's comeback after having a baby.
In the deciding mixed doubles, Kerber and Zverev combined to beat Angelica Moratelli and Sonego 6-3, 6-0.
"It was a special day for me, coming back after so long, playing singles and having such a high level in doubles," Kerber said.
"It was great to get my first win since coming back - it feels great."
Casper Ruud, the top-ranked Norwegian in history after reaching a career-high two by making the 2022 US Open final, was a level above Dutchman Tallon Griekspoor in a 6-3, 6-4 win at Sydney.
He converted three of seven break points to clinch victory in 72 minutes against a player who has risen to 23 in the world on the back of ATP Tour titles in Pune and Hertogenbosch this year.
It helped Norway level the tie after the Netherlands' Arantxa Rus downed Malene Helgo 7-6 (7/4), 6-1 in the women's singles.
Ruud, now ranked 11, returned to partner Ulrikke Eikeri in the mixed doubles, but they were unable to overcome Demi Schuurs and Wesley Koolhof who rallied from a break down in each set to win 7-6 (7/5), 7-5.
