Alexander Zverev and Stefanos Tsitsipas opened their seasons with fighting wins in the ATP Cup Wednesday as they warm up for the Australian Open, while Italy and Russia became the first teams into the semi-finals.

Seventh-ranked Zverev had to dig deep to overcome Canadian dangerman Denis Shapovalov 6-7 (5/7), 6-3, 7-6 (7/4) and seal the tie for Germany.

But he gained valuable court time ahead of another crack at a first Grand Slam trophy at the Australian Open starting next week.

Zverev, who fell agonisingly short in the US Open final last year to Dominic Thiem, said having his brother Mischa as team captain had helped him.

"All the players enjoy having him as captain, he's been fantastic in training as well. It's just great to have him here," he said.

Germany's Jan-Lennard Struff upset Milos Raonic 7-6 (7/4), 7-6 (7/2) in the opening match and Canada are now out after a second narrow loss for Shapovalov, who pushed Novak Djokovic hard in their Group A clash on Tuesday.

Djokovic's defending champions Serbia beat Canada 2-1 in their opener on Tuesday and will play Germany on Thursday for a place in the semis.

Greece's world number six Tsitsipas was also forced to work hard by Australian Alex de Minaur, who won the season-opening Antalya Open last month, eventually triumphing 6-3, 7-5.

Tsitsipas, a semi-finalist at the Australian Open in 2019, said he gave it his all.

"It was a thrilling match, both of us gave everything we had," he said.

"I managed to stay calm and resilient. I just tried to take my opportunities and press as much as I could."

Australia's John Millman had won the opening singles match against Michail Pervolarakis, with the Group B tie sent into a deciding doubles.

Berrettini delivers

Italy became the first nation into the last four by beating France to follow their opening win against Austria in Group C.

Fabio Fognini won just five games in his ATP Cup opener on Tuesday against Dennis Novak, but bounced back to battle past Benoit Paire 6-1, 7-6 (7/2) and give Italy a 1-0 lead.

Matteo Berrettini, tipped by Djokovic as among the new generation capable of winning a Grand Slam, clinched the tie with a comfortable 6-4, 6-2 victory over Gael Monfils, ranked a place below him at 11.

"I was ready for any score. I kept going," said Berrettini, who recovered from going down an early break in the first set. "I thought about my weapons and that's how I won the match."

"It's a great team, great guys. We are enjoying on court, but off court as well," he added. "I think when you're playing for your country, for your team, it's something special."

Russia, the only team to feature two top-10 players, also made the semis, proving too strong for Japan.

World number eight Andrey Rublev crushed Yoshihito Nishioka in just 64 minutes, losing just seven games across his two singles encounters at this year's event, before fourth-ranked Daniil Medvedev cruised past Kei Nishikori 6-2, 6-4.

"Really happy for the team," said Medvedev. "Semi-finals is a big step ... really happy that we made it in both singles."

Serbia overcame Spain in the final of the inaugural and hugely popular tournament last year in Sydney -- launched as a rival to the Davis Cup.

It has been slimmed down from 24 teams to 12 this year due to the coronavirus over five days at Melbourne Park with $7.5 million at stake, rather than the multi-city format employed in 2020.