U.S. 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.

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 was set to team up with Pospisil to face Marcel Granollers and Pedro Martinez in the doubles decider.