Defending champion France opened up a 2-0 lead over Spain while Croatia dominated United States on the opening day of the Davis Cup semi-finals.
Debutant Benoit Paire rewarded captain Yannick Noah's faith by claiming the opening point for France in a hurry, thrashing an injured Pablo Carreno-Busta in straight sets.
Picked to play singles for the first time with both Gael Monfils and Jo-Wilfried Tsonga not available, Paire, 29, eased to a 7-5 6-1 6-0 triumph.
Carreno-Busta revealed after the heavy defeat he had suffered a recurrence of the left-thigh injury that hampered him at the US Open.
"I started to feel the adductor problem again and at 3-0, 4-0 in the second set I knew it was going to be impossible to win," he told the Davis Cup's official website.
A Spain side minus the services of Rafael Nadal were pushed to the brink of defeat when Roberto Bautista Agut came up short in a tense clash with Lucas Pouille that went the distance.
Pouille - who beat Belgium's Steve Darcis in the decisive fifth rubber in the final last year - lost the first set before letting slip a 2-1 lead, though he claimed the crucial break in the ninth game of the decider before serving out for a 3-6 7-6 (7-5) 6-4 2-6 6-4 victory.
Host Croatia WAS dominant in both singles encounterS in Zadar, cruising to a 2-0 advantage without dropping a set.
Borna Coric came through a tense tie-break in the second set to record a 6-4 7-6 (7-4) 6-3 result over Steve Johnson after two hours and 24 minutes on court.
Marin Cilic doubled his nation's lead as he gave Frances Tiafoe a tough Davis Cup baptism, beating the debutant 6-1 6-3 7-6 (7-5) to pick up a landmark point in his career.
The 2014 US Open champion has now won 37 matches in the competition, more than any other Croatian having previously sat level with Ivan Ljubicic.
"In these later stages of my career, I enjoy so much to play in the Davis Cup," Cilic said. "We have great success, great chemistry in the team."
His contest with Tiafoe was officiated by Carlos Ramos, the umpire returning to work for the first time since the controversial women's final at the US Open.
Serena Williams branded Ramos a "liar" and a "thief" during her defeat to Naomi Osaka, with the American fined $17,000 for three code violations.