PIX: Alcaraz loses in first match as world No.1

US Open champion Carlos Alcaraz crashed to defeat on his debut as world number one, losing 6-7(3), 6-4, 6-2 to Canada's Felix Auger-Aliassime in their Davis Cup Finals group-stage clash on Friday.

The 19-year-old Spaniard, who became the youngest player to top the ATP rankings when he claimed the title at Flushing Meadows on Sunday, joined Sergi Bruguera's squad on Tuesday and did not play in their opening 3-0 victory over Serbia.

"I didn't arrive in very good physical condition. Very, very tough. The court is very slow. I had just two days to adapt my game to this court. It was really tough day," Alcaraz said.

Taking to the court after Roberto Bautista Agut beat Vasek Pospisil to put Spain up 1-0, Alcaraz sealed a tight opening set in the tiebreak, before his 13th-ranked opponent switched gears.

"It's a big win for me and for the team," Auger-Aliassime said. "For me, because Carlos is the new number one. But he made a big effort, so we have to pay him respect for flying across the Atlantic and coming here to play... credit for that.

"I think I showed at the end I was a bit better in the third set and I gave it all."

The 22-year-old Auger-Aliassime's first win over a world number one dragged his team back into the tie at 1-1 and he then teamed up with Pospisil to face Marcel Granollers and Pedro Martinez in the doubles decider.

The Canadian pair came back from a break down at 4-5 in the deciding set to win the last three games and clinch the rubber and the tie with a 4-6, 6-4, 7-5 victory.

The victory took Canada to the top of group B with Spain, in second spot, Serbia and South Korea all with a chance to qualify for the knockout stages in Malaga from November 22-27. Two teams from each of the four groups will advance to the next stage.