World number one Rafael Nadal won a battle of attrition against fellow left-hander Yoshihito Nishioka of Japan 7-6(4), 6-4 to seal Spain's spot in the final eight of the inaugural ATP Cup in Perth on Wednesday.
Meanwhile his long-time rival, world number two Novak Djokovic, had an easier outing in Brisbane, strolling past Chile's Cristian Garin 6-3, 6-3 to stay undefeated in the new year and to ensure Serbia will travel to Sydney for the knockouts having won all three of their ties.
Nadal had to twice fight back from a break down against the Japanese world number 72 in the opening set before winning the tiebreaker while the Spaniard broke his opponent's serve to go up 5-4 in the second before holding serve for victory.
"It's the first day I was playing under such heavy conditions, two previous days I had played in the night. The feeling on court was completely different," Nadal said in an on-court interview.
"I played against an opponent who started the season on fire, winning two matches against two great players with very comfortable results. Today was a very, very tough one."
It was Nadal's third singles win in the $15 million ATP Cup, which features 24 nations competing in Perth, Brisbane and Sydney over 10 days.
All six group winners and the two best second-placed teams will advance to the final eight stage in Sydney. Besides hosts Australia, Serbia, Britain, Russia and Spain have qualified.
Garin broke Djokovic in the second set to go up 3-1 but it seemed to bruise the seven-times Australian Open winner's ego as he won the next five games to complete victory.
"Sydney's got a big Serbian community, hopefully the support can be as good as it was here, even better," Djokovic said.
Poland's Hubert Hurkacz also picked up his third singles win of the tournament with an upset 3-6, 6-4, 7-6(5) win over world number four Dominic Thiem of Austria in Sydney.
It was the second loss in the new season for Thiem, who also lost to Croatia's Borna Coric in his opening match.
Kyrgios keeps calm to outlast Tsitsipas
Australian Nick Kyrgios outlasted world number six Stefanos Tsitsipas 7-6(7), 6-7(3), 7-6(5) after an intense duel to seal the tie for the hosts against Greece at the inaugural ATP Cup team event in Brisbane on Tuesday.
Kyrgios slammed a backhand winner on matchpoint and fell on his back to celebrate his second win in as many meetings against the ATP Finals champion as the partisan crowd at the Pat Rafter Arena roared on.
His win ensured Australia will go to the knockouts having won all their ties in the $15 million ATP Cup, which features 24 nations competing in Perth, Brisbane and Sydney over 10 days.
"Exhausting! It was lot of fun. Knew it was going to be a tough match," Kyrgios said in a courtside interview.
"He's an amazing player and he's so young. He's going to do some special things in the sport."
Kyrgios had missed the tie against Canada with a back problem and on Tuesday replaced Australia number one Alex de Minaur, who sat out with an abdominal strain.
While Kyrgios, who has been placed on probation by the governing body of men's tennis over his behaviour, losing his cool on court is not surprising, it was Tsitsipas who was docked a point for hitting a ball at the Greek Team Zone.
The 21-year-old also broke his racquet in frustration after losing the opening set which earned him a berating from his mother, who was present in the team zone.
Kyrgios stayed unfazed and kept his focus on his own game, which consisted of his usual through-the-leg tweeners, no-look drop shots and one underarm serve.
"I am on probation, so I got to stay in check. I'm trying, I'm trying (to be a good boy)," he said as the crowd clapped in appreciation.
Canadian Denis Shapovalov earlier picked up his second win against a top-10 player with a 6-2, 6-2 thumping of Alexander Zverev as the German continued to struggle with his serve ahead of this month's Australian Open.
Shapovalov, 20, needed just 70 minutes to brush aside world number seven Zverev, who lost his third straight singles before the year's first Grand Slam starting in Melbourne on Jan. 20.
Zverev, the 2018 ATP Finals champion, served seven double faults against Shapovalov to take his total for the tournament to 31.
"There's a lot of things that I still need to improve, but it's the start of the season," Zverev said. "I need to find my rhythm."
Shapovalov, who beat Tsitsipas in his opening match, returned with team mate Felix Auger-Aliassime to win the deciding doubles rubber against Kevin Krawietz and Andreas Mies 6-3, 7-6(4), earning Canada a 2-1 win in the tie to keep them in the hunt for the quarter-finals.
All six group winners and the two best second-placed teams will advance to the final eight in Sydney. Besides the hosts, Serbia, Britain and Russia have already qualified.
Russian Daniil Medvedev, the US Open finalist and world number five, won his third straight singles rubber in Perth, beating Norway's Casper Ruud 6-3, 7-6(6).