Raucous home support for holders Argentina did not make up for the absence of Juan Martin del Potro and Federico Delbonis on Friday as they fell 2-0 behind to Italy in the first round of the Davis Cup.
Croatia, runners-up last year, were without their top man Marin Cilic but managed to end the opening singles level 1-1 with a Spanish team lacking Rafael Nadal after Franko Skugor upset Pablo Carreno Busta over five sets.
Only Serbia, champions in 2010 and facing Russia at home in Nis, had the services of a top 10 player in Novak Djokovic and they took a 2-0 lead.
Djokovic, who readjusted his start to the season after a shock early exit from the Australian Open, had a scare when he dropped the first set to Daniil Medvedev but prevailed 3-6, 6-4, 6-1, 1-0 when the young Russian retired.
Viktor Troicki had earlier beaten Karen Khachanov 6-4, 6-7(3), 6-3, 1-6, 7-6(6).
Del Potro, like most of tennis’s leading men, had announced before the season began he would not be available for the opening round of the World Group.
This left top seeds Argentina, a British team without Andy Murray and the Swiss minus Roger Federer and Stan Wawrinka vulnerable to defeat.
Argentina won away in every round last year, but in Buenos Aires Guido Pella lost 6-3, 6-3, 6-3 to Paolo Lorenzi and Andreas Seppi beat Carlos Berlocq 6-1, 6-2, 1-6, 7-6(6) to leave the holders on the brink of elimination.
"For the first time I felt uncomfortable playing the Davis Cup," Pella, who had last helped Argentina upset Britain in the 2016 semi-final in Glasgow, told reporters.
Croatian Skugor, ranked nearly 200 places below Carreno Busta, won 3-6, 6-3, 6-4, 4-6, 7-6(6) in just over four hours in Osijek before Spain recovered with Roberto Bautista Agut’s 6-4, 6-2, 6-3 win over Ante Pavic.
France will go into Saturday’s doubles 2-0 up against a Japanese team without Kei Nishikori after three-set wins for Richard Gasquet and Gilles Simon in Tokyo.
Talented German teenager Alexander Zverev won his first Davis Cup point when he beat Belgian debutant Arthur de Greef 6-3, 6-3, 6-4 to level their tie at 1-1 in Frankfurt.
Earlier, Steve Darcis had come back from 3-0 down in the fifth set to beat Phillip Kohlschreiber 6-4, 3-6, 2-6, 7-6(2), 7-6(5).
Britain, facing a Canada team without Milos Raonic in Ottawa, made a good start when Dan Evans beat Denis Shapalov 6-3, 6-3, 6-4 but Vasek Pospisil hit back to level the tie with a 6-4, 6-1, 7-6(3) win over Kyle Edmund.
Jack Sock put the United States on the front foot against Switzerland by beating Marco Chiudinelli 6-4, 6-3, 6-1 in the first rubber.
Big-serving John Isner later put down Henri Laaksonen 4-6, 6-2, 6-2, 7-6(1) to double the 32-times champions' lead, the American serving up his 500th ace on Davis Cup duty in the process.
"Jack did a really great job and took the pressure off me," said Isner. "We're going to come out here tomorrow and try and finish it off."
Late night Friday in Alabama bumps into Saturday morning Down Under and soon after Isner closed day one in Birmingham, Australia became the first team into the quarter-finals when they took an unassailable 3-0 lead over the Czech Republic.
Debutant Jordan Thompson and Nick Kyrgios had given the 28-times champions a 2-0 lead with singles victories at Kooyong on Friday and Sam Groth and John Peers defeated Jan Satral and Jiri Vesely 6-3, 6-2, 6-2 in the doubles to seal the win.