Spain beat Serbia in their opening tie of the Davis Cup group stage on Wednesday as US Open champion Carlos Alcaraz watched from the sidelines, but Great Britain and Andy Murray lost to the United States.
Alcaraz, who made his Davis Cup debut in a qualifying tie against Romania in March, arrived in Valencia only on Tuesday after becoming the youngest men's world number one in history with Sunday's triumph in New York.
With Rafael Nadal also absent against a Serbia team missing Novak Djokovic, Albert Ramos-Vinolas won the opening rubber against Laslo Djere 2-6, 7-6 (7/5), 7-5.
Roberto Bautista Agut secured the Group B tie for Spain by defeating Miomir Kecmanovic 7-6 (7/5), 7-6 (7/5) in the second singles.
"The best experiences of my life playing tennis have been in this competition," said Bautista Agut.
"Today I really enjoyed playing in front of the Spanish crowd in Valencia, next to my home town. I'm exhausted because it was a tough match and we have a lot of work (still) to do this week."
Marcel Granollers and Pedro Martínez sealed a 3-0 victory when they won the doubles against Nikola Cacic and Dusan Lajovic 6-7 (5/7), 6-2, 6-2.
Jack Sock and Rajeev Ram of the United States sealed a 2-1 win against Great Britain in Group D in Glasgow, edging Murray and Joe Salisbury 5-7, 6-4, 7-5 in a thrilling doubles rubber that finished well after midnight.
"We were playing against one of the biggest legends of the game, playing in his home country," said Sock of facing Murray.
"It was an electric atmosphere. We stuck it out and got the win."
Ram said it had not been easy being on the other side of the net from his regular doubles partner, Salisbury.
"It's tricky, we've been playing together for four years. But we go out and compete as best as we can," said Ram. "Playing for our country is huge for us."
Earlier Cameron Norrie, the world number eight, overcame 12th-ranked Taylor Fritz 2-6, 7-6 (7/2), 7-5 to bring Britain level in the tie after Tommy Paul took down Dan Evans in three sets in the opening rubber.
The late finish meant the US players would have only a few hours rest until they were due back on court on Thursday to face Kazakhstan.
"These guys gave everything, Team GB gave everything," said stand-in American captain Bob Bryan.
"It's 0100 on a Thursday. We've got to go again in 13 hours. Kazakhstan won't be any easier."
Lorenzo Musetti and Matteo Berrettini gave Italy the perfect start to their campaign with victory over Croatia in Bologna.
The 20-year-old Musetti saw off Borna Gojo 6-4, 6-2 in the Group A tie.
Former Wimbledon runner-up Berrettini had to fight back from a set down against Cincinnati Masters champion Borna Coric, but won 6-7 (4/7), 6-2, 6-1.
Simone Bolelli and Fabio Fognini edged a final-set tie-break against Nikola Mektic and Mate Pavic to complete a 3-0 sweep.
Italy have high hopes of claiming only a second Davis Cup title and first since 1976.
Their top-ranked player Jannik Sinner could yet be ready to play against Argentina on Friday.
Germany began Group C with a first Davis Cup win over France since 1938 after Tim Puetz and Kevin Krawietz defeated Nicolas Mahut and Arthur Rinderknech in the doubles in Hamburg.
"Making positive history is always great," said Germany captain Michael Kohlmann, whose country had lost the previous eight meetings with the French.
"Today was like a roller coaster, but in the end it's a big win. It is huge for us."
Jan-Lennard Struff beat Benjamin Bonzi 6-4, 2-6, 7-5 to give the home team the lead, before Adrian Mannarino levelled with a 6-4, 6-3 win over Oscar Otte.