Daniil Medvedev and Matteo Berrettini both slumped to shock defeats in their opening matches of the season at the ATP Cup on Sunday, but there were no mistakes from Alexander Zverev.

US Open champion Medvedev is leading a Russian team in Sydney depleted by injuries and Covid-19 as they look to defend the teams title won against Italy last year.

But the world number two, who captured four tour-level crowns in 2021, was stunned by impressive 35th-ranked Frenchman Ugo Humbert despite leading by a set and 3-0.

The Frenchman rallied to pull off a big upset 6-7 (5/7), 7-5, 7-6 (7/2) in a gruelling 2 hours 55 minutes as Medvedev, red-faced from the heat, ran out of steam.

"It was very tough today, but I'm very happy," said Humbert after one of the biggest wins of his career.

Russia's world number two Daniil Medvedev (pictured) was stunned by France's Ugo Humbert
Russia's world number two Daniil Medvedev (pictured) was stunned by France's Ugo Humbert AFP / PIERRE-PHILIPPE MARCOU

"I had some opportunities in the first set, and I just tried to stay relaxed, stay focused on what I had to do and it was a great match."

With fifth-ranked Andrey Rublev missing after contracting coronavirus at an Abu Dhabi exhibition last month, unheralded Roman Safiullin was drafted in as Russia's second singles player.

He saved 15 of 19 break points to earn a 2-6, 7-5, 6-3 win over France's Arthur Rinderknech to set the stage for Medvedev, who could be the top seed at this month's Australian Open if defending champion Novak Djokovic fails to show up.

Djokovic has refused to confirm if he has been vaccinated against Covid-19, a requirement at the opening Grand Slam of the year, and pulled out of the ATP Cup.

The tie went to a deciding doubles rubber with Medvedev backing up with Safiullin to win and seal the points.

Russia are in a tough group that also includes hosts Australia and 2021 runner-up Italy, led by world number seven Berrettini, who was also beaten, crashing to 34th-ranked Alex de Minaur 6-3, 7-6 (7/4).

Alexander Zverev enjoyed a comfortable ATP Cup win
Alexander Zverev enjoyed a comfortable ATP Cup win AFP / William WEST

"I've worked really hard to be here, I'm in the best shape of my life and ready to have a big year," said de Minaur, in front of a noisy home crowd.

Earlier, Italian number two Jannik Sinner crushed Australia's Max Purcell 6-1, 6-3.

Only the winning nations progress to the semi-finals from each of the four groups in the event, which runs until January 9.

In the other night match, Zverev's Germany were up against a British team led by Cameron Norrie, who had a breakout 2021 season that saw him storm through the rankings to end at 12.

But he was no match for the world number three, who shook off first set rust to overwhelm the Briton 7-6 (7/2), 6-1.

"It's the first match of the season and it's never going to be the best match of your life, but I'm happy," said Zverev, who captured his second ATP Finals title in November -- his sixth trophy of the year.

Britain's Dan Evans won the opening singles match against Germany's Jan-Lennard Struff 6-1, 6-2.

The United States, meanwhile, won their clash against Canada, with big-serving veteran John Isner thrashing Brayden Schnur 6-1, 6-3 after he stepped in at short notice for 14th-ranked Denis Shapovalov, who is still fatigued after testing positive for Covid-19 on his arrival in Sydney.

"I'm disappointed not to be able to play the first match, but I just didn't feel ready physically yet," said Shapovalov, who later eased his way back by playing the dead doubles rubber.

American Taylor Fritz earlier upset 11th-ranked Canadian Felix Auger-Aliassime 6-7 (6/8), 6-4, 6-4.