Russian tennis star Daniil Medvedev's suffered a shock defeat against Frenchman Ugo Humbert to open his 2022 season at the ATP Cup on Sunday.

The world No. 2 had been expected to seal the Group B tie for Russia after Roman Safiullin beat Arthur Rinderknech in the opening singles match, but Humbert's 6-7(5) 7-5 7-6(2) victory after a three-hour dogfight kept France alive at 1-1 going into the doubles.

Medvedev, however, changed his shirt and returned to the court alongside Safiullin to clinch the tie for the defending champions with a 6-4 6-4 win over Fabrice Martin and Edouard Roger-Vasselin.

"I'm feeling very bad physically, that's all I can say," Medvedev laughed.

That was a little surprise after his titanic contest with Humbert, who was in inspired form and clung on tenaciously whenever it looked like Medvedev might get away from him.

Medvedev squeezed through the first set in a tiebreak but had to go the distance after giving up a 3-0 lead to ceding the second set to the 35th ranked Frenchman.

The tiring Russian looked to have run out of patience in the final tiebreaker and Humbert secured the biggest win of his career on his second match point with a rasping backhand return. "I'm not feeling very good now but I'm very happy," Humbert laughed. "It was a great match."

Earlier, Canadian Denis Shapovalov was a late withdrawal from his singles match against American John Isner in Sunday's other early tie at the $10 million men's team tournament.

The world No. 14 had been forced to isolate in his hotel room after a positive COVID-19 test upon arrival in Sydney.

"I just didn't feel ready physically yet, so we did what is best for the team," said Shapovalov, who later took to the court to play doubles with Felix Auger-Aliassime.

That was a dead rubber, however, as Isner had beaten Shapovalov's singles replacement Brayden Schnur before Taylor Fritz downed Auger-Aliassime to give the U.S. a 2-0 lead in the Group C clash.

Isner and Fritz also won the doubles rubber to put a 3-0 gloss on their nation's first ATP Cup tie victory.

Host Australia suffered a blow ahead of its opening Group B tie against Italy later Sunday when James Duckworth was forced to withdraw with a hip problem. Germany faces Britain in Group C in the other evening tie.