US Open champion Carlos Alcaraz lost on his debut as world number one, getting beaten 7-6(7-3), 4-6, 2-6 to Canada's Felix Auger-Aliassime in their Davis Cup Finals group-stage clash on Friday. The 19-year-old Spaniard, joined Sergi Bruguera's squad on Tuesday and did not play in their opening 3-0 victory over Serbia.