Greek star Stefanos Tsitsipas has complained of an arm irritation after Denis Shapovalov pulled off the ATP Cup's first upset to seal victory for Canada in their opening clash in Brisbane on Friday.
The big serving left-hander upstaged world No.6 Tsitsipas 7-6 (8-6) 7-6 (7-4) to give Canada an unbeatable 2-0 lead in their tie before securing a clean-sweep with a doubles win in the inaugural 28-team tournament to be played across Brisbane, Sydney and Perth.
In Perth the United States have been forced into a live doubles rubber against Norway, Casper Ruud upstaging John Isner 6-7 (7-3) 7-6 (10-8) 7-5 after Taylor Fritz had eased by world No.332 Viktor Durasovic 6-2 6-2.
And in Sydney, Belgium had to dig deep to take a 2-0 lead over an unheralded Moldova at Ken Rosewall Arena.
All eyes were on Tsitsipas, 21, who arrived at Pat Rafter Arena as tennis' hottest property after becoming the youngest person in 18 years to clinch the ATP Finals in November.
However, world No.15 Shapovalov had the measure of the 2019 Australian Open semi-finalist to extend his career head to head record to 3-1.
Shapovalov, 20, fired down 12 aces to the Greek's two and thrashed 32 winners compared to his opponent's 19 in their clash which lasted more than two hours.
"He had an unbelievable end to the season and he's definitely one of the top players in the world right now," the Canadian said.
"So to beat a guy like this first match of the year, it's really special for me."
Shapovalov's momentum was only stalled when a line umpire tripped and injured herself mid-point in the second set and had to be helped off the court in a wheelchair.
Tsitsipas had his own injury concerns though.
"I didn't feel good, to be honest," he said.
"I woke up a bit sore in my arm and I was being a little bit stressed before the match because I didn't know if I'm going to be able to serve, but I served okay.
"My fighting spirit and the soul I put on the court is more important than - injuries can also affect your performance - but it didn't quite affect it today too much."
Still buzzing from featuring in their maiden Davis Cup final in November, Canada had a dream start when world No.21 Felix Auger-Aliassime overpowered Michail Pervolarakis - ranked 466 places lower than his opponent - in straight sets in little over an hour.
Hosts Australia - led by world No.18 Alex de Minaur - take on Germany in Friday's evening session in Brisbane.
Moldova were clearly unruffled after the Romanian anthem was incorrectly played at the opening ceremony as they came out swinging against an unsuspecting Belgium.
World No.818 Alex Cozbinov played the match of his life before going down in three sets spanning more than three hours to Belgian Steve Darcis, ranked 661 places higher.
David Goffin then gave Belgium a 2-0 lead, downing Moldova's top seed and world No.46 Radu Albot 6-4 6-1.
Great Britain will battle Bulgaria in Sydney on Friday night while Russia meets Italy in Perth.