Iga Swiatek stormed to a commanding victory in her season opener at the United Cup on Monday, while Stefanos Tsitsipas faltered as Kazakhstan stunned Greece to secure a spot in the knockout round of the mixed-team tournament.
The five-time Grand Slam champion returned to action for the first time since it was revealed in late November that she had served a one-month doping ban.
Showing no signs of nerves, the Polish star dominated in Sydney, delivering a resounding 6-1, 6-0 win over Norway’s Malene Helgo.
"I'm pleased with everything, honestly. I'm happy, for sure, with the performance," said the world No. 2.
"It's not always easy to play for your country; sometimes the pressure is a bit bigger.
"I feel good, I feel happy, happy I can play tennis overall," she added. "I'm just going step by step, and we'll see what's next."
The 23-year-old tested positive for the heart medication trimetazidine (TMZ) in an out-of-competition sample in August when she was ranked No. 1.
However, the International Tennis Integrity Agency (ITIA) accepted that the violation was not intentional, and she escaped with a one-month sanction.
While Swiatek was in electric form, Greek star Tsitsipas had a day to forget.
Coming off a mediocre 2024, in which his ranking dropped from a career-high No. 3 to No. 11, he was hoping for a confidence-boosting fresh start in Perth.
But he was stunned 6-4, 7-6 (0) by 78th-ranked Alexander Shevchenko.
Kazakh world No. 6 and 2022 Wimbledon champion Elena Rybakina then survived a late wobble to bury Greek hopes with a 6-4, 6-3 defeat of Maria Sakkari.
"I'm really happy for the team," said Rybakina, now working with Novak Djokovic's former coach Goran Ivanisevic. "We all tried hard.
"It was a tough match for me today, and I'm so happy to be in the quarterfinals."
Rybakina took the first set and led 5-0 in the second, but former world No. 3 Sakkari won three games in a row to threaten a comeback.
Rybakina came through on her third match point as Sakkari mishit an overhead.
"At 5-0, you have nothing to lose. She played more aggressively and hit some good shots," Rybakina said.
Shevchenko called his win over a sluggish Tsitsipas "one of the best" of his career.
"He was not at his best today. But I played really well and found the moment to win that match," he said. "It will definitely go in my record as one of my best wins."
Defending champions Germany, led by Alexander Zverev, defeated China 2-1 in the Perth evening session as both nations reached the quarterfinals.
The Germans topped their group and will face Kazakhstan on Wednesday. China advanced as the best runner-up in Perth.
Second-ranked Zverev recovered from a poor start to beat Zhang Zhizhen 2-6, 6-0, 6-2, but Gao Xinyu stunned Germany's Laura Siegemund 6-1, 3-6, 6-3 to even the tie.
The German pair combined in mixed doubles to defeat Zhang Shuai and Zhang Zhizhen 6-2, 7-6 (3).
Zverev struggled early in his opening match against the inconsistent Zhang, who dominated the first set but faded quickly.
"I didn't start off well. I was missing more shots from my backhand side, which is normally the most stable," Zverev said. "He was also playing extremely well, so aggressively. He was not letting me into the match or find my rhythm.
"Then I found my rhythm, and I'm happy with my performance in the second and third sets."
In Sydney, Katie Boulter led Britain to victory over Argentina, with all three teams in the group, including hosts Australia, still in contention for the knockout rounds.
World No. 24 Boulter defeated 100th-ranked Nadia Podoroska 6-2, 6-3.
Underdog Billy Harris looked set to seal the tie for Britain after taking the first set against Tomas Martin Etcheverry.
However, the Argentine rallied for a gutsy 3-6, 6-3, 6-2 win, sending the tie to a deciding mixed doubles match.
Etcheverry returned with Maria Carle to face Boulter and Charles Broom, but the British duo prevailed 7-6 (4), 7-5.
