Earlier, Vancouver's Vasek Pospisil recorded his second major upset in as many days when the world No. 150 beat No. 36 Reilly Opelka 7-6 (5), 7-6 (7).
"Canada's really in the best place it's been in tennis history," said Pospisil. "We have incredible young talents … and then some older guys, which I guess I can say I'm older now, myself now and Milos [Raonic].
"Right now we're on a run and we have tons of great players. It's just how it goes."
Following the clinching singles match from Shapovalov, Canada swapped out Pospisil for Felix Auger-Aliassime to play doubles. The young duo had previously shared the court for Canadas first Davis Cup Juniors championship in 2015. However, they would not get a chance to play together again this year, as Canada gave the U.S. a walk-over, ending the match, 2-1.
Later, Americans Sam Querrey and Jack Sock took the doubles match against Shapovalov and Felix Auger-Aliassime in a walkover.
🇨🇦 Vasek does it again! 🇨🇦@VasekPospisil takes down another major @DavisCupFinals threat in Reilly Opelka, claiming the 1-0 lead for Canada over the USA with a 7-6(5), 7-6(7) victory. #GoCanada | #AnythingIsPospisil pic.twitter.com/bWTCGDjqE2
Canada (2-0) has now clinched Group F after winning all four of its singles matches against the Americans and Italy.
It was Canada's first win against the U.S., in 16 Davis Cup ties. Canada had a match record of 3-70 against the Americans entering Tuesday.
The 29-year-old Pospisil, from Vancouver, beat world No. 12 Fabio Fognini on Monday en route to Canada's 2-1 win over Italy. Pospisil was a late replacement for Felix Auger-Aliassime (recovering from an ankle injury) against Italy.
The six-foot-11 Opelka, known for his big serve, fought off one match point against Pospisil in the second set to tie it at 6-6. But Pospisil rebounded to end it in two sets.
Pospisil, who reached a career-high No. 25 in the rankings in 2014, tumbled down the list after missing the first half of 2019 following back surgery.
But he has shown signs of progress in recent months, upsetting then-No. 9 Karen Khachanov at the U.S. Open before advancing to the round of 16 at the Shanghai Masters.
Playing a level down in Challengers earlier this fall, Pospisil captured two tournament titles in the U.S.
"To be honest, I just wanted to finish the year healthy, that was my goal when I came back," said Pospisil. "I mean, I had surgery in the lower back. I didn't know, I didn't even know if I'd be, like, get to this level. I mean, I was optimistic, but I'm just saying you never know, right?"
Shapovalov, 20, also has been on a roll. He captured his first career ATP Tour title at the Stockholm Open last month before reaching the final of the Paris Masters.
"I think the last two matches I've been able to really play well. Aside from struggling with the serve a little bit, I think off the ground, off the returns, I played really well and kept the form from Paris," said Shapovalov.
The winners of each of the six groups and the next two best teams advance to the quarterfinals. Canada will face either Belgium or Australia in the next round.
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Should Canada advance to the semi-finals, it would mark only the third time in Davis Cup history (1913, 2013).
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For the first time in the 119-year history of the Davis Cup, which is effectively the World Cup of men’s tennis, Canada has defeated the United States. Canada previously had a 0-15 all-time record against its neighbours to the south.
They did so without the services of two of our country’s best players as well, as world No. 21 Felix Auger-Aliassime and Milos Raonic (No. 31) didn’t play due to injury.
Denis Shapovalov (No. 15) clinched it for Canada with a straight sets victory (7-6, 6-3) over No. 32-ranked American Taylor Fritz. Shapovalov’s win followed up Vasek Pospisil’s surprise victory (7-6, 7-6) over No. 36-ranked Reilley Opelka.
“It’s amazing, honestly it’s just such an honour to play for my country, represent it,” said Shapovalov. “I think the whole team is putting such an amazing effort. The fans are spectacular, it’s not just me, it’s the whole team and everyone around us.”
Canada are the first team to make it through to the quarter-finals! 🇨🇦👏#DavisCupMadridFinals #byRakuten pic.twitter.com/zroHthd29r
The win by Canada secured first place in Group F and puts them through to the quarter-finals on Thursday.
Shapovalov and Pospisil each scored upset wins on Monday to defeat Italy. Shapovalov took down world No. 8 Matteo Berrettini in three sets (7-6, 6-7, 7-6), while Pospisil defeated 12th-ranked Fabio Fognini (7-6, 7-5).
Both wins came in front of hundreds of Canadian tennis fans who have made the trip to Spain for the week-long event. This is the first year of the new Davis Cup format, which has streamlined its competition. ‘Ties’ are now a best-of-three rubbers format, consisting of two singles and one doubles match. The Finals replaces the World Group, with 18 countries competing in the Finals at a central location.
🇨🇦 @TennisCanada is READY to rumble!!! 🙌🏻🥁🎺#DavisCupMadridFinals #byRakuten pic.twitter.com/L8as7E21ZX
Waiting for Canada in the quarters will be the winner of Group D — either Australia or Belgium. The Canadians could get a boost to their lineup, as Auger-Aliassime is nearing a return from an ankle injury.