Playing without stars Roger Federer and Stan Wawrinka, defending champion Switzerland slumped to a first-round loss to Belgium in the Davis Cup on Sunday, while Andy Murray scored the decisive point to lift Britain past the United States and Novak Djokovic led Serbia to victory over Croatia.
After Federer and Wawrinka led Switzerland to its first Davis Cup title last year, the two top-10 players decided not to play Davis Cup this year, leaving Switzerland with a diminished team - and chances - against Belgium.
Trailing 2-1 heading into the reverse singles on Sunday, Henri Laaksonen, ranked No. 344, managed to keep the Swiss alive by beating Belgium's Steve Darcis 6-3, 3-6, 3-6, 7-6 (5), 6-1 in the first match to level the tie at 2-all.
The 21st-ranked David Goffin then shook off a back injury to beat No. 321 Adrien Bossel 6-4, 6-0, 6-4 and send Belgium into the quarterfinals. Goffin made light work of the contest, hitting 15 aces and breaking Bossel's serve five times.
"It was tough to play the deciding match," Goffin said. "But at the end I was really happy to win that last point."
It was just the ninth time since 1972 that the defending champions had fallen in the first round the following year.
Elsewhere, Andy Murray beat John Isner 7-6 (4), 6-3, 7-6 (4) to clinch Britain's first-round victory over the United States, jumping up and down with his jubilant teammates in a celebration huddle after the match in Glasgow, Scotland.
It was the second straight year that Britain has ousted the Americans at this stage in the World Group.
"We are playing at our level, at our limits right now," said Murray, who beat Donald Young in the opening singles match Friday. "Hopefully we can do that right through to the end of the year."
Britain won the last of its nine Davis Cup titles in 1936, and hasn't reached the semifinals since 1981.
U.S. captain Jim Courier said Britain deserved to be among the favorites.
"When you have a great player and champion like Andy, against most teams they are up 2-0 going in," Courier said.
Britain next hosts France, which closed out its first-round tie against Germany with a victory in the doubles on Saturday, giving it an unassailable 3-0 lead. The Germans won the dead rubbers on Sunday to make the final score 3-2.
Serbia won both reverse singles on Sunday for a 5-0 victory against Croatia. Djokovic won the opening singles match against Mate Delic on Friday, then teamed with Nenad Zimonjic to clinch the tie with a doubles victory on Saturday.
"We hoped Novak Djokovic wouldn't play, but that was just wishful thinking," Croatian coach Zeljko Krajan said. "Everything was great, except for the players, who were ruthless."
Serbia will play either Argentina or Brazil in the quarterfinals.
Argentina's Leonardo Mayer defeated Joao Souza in the longest Davis Cup singles match ever on Sunday, converting his 11th match point to win 7-6 (4), 7-6 (5), 5-7, 5-7, 15-13 and keep the hosts' hopes alive by leveling the tie at 2-all.
Mayer needed 6 hours, 42 minutes to beat Souza, which topped the previous longest singles match - John McEnroe's win over Mats Wilander in 1982, which lasted 6 hours, 22 minutes.
After squandering 10 match points, Mayer finally broke Souza's serve while leading 14-13 in the fifth set. The Argentine dropped to the ground as the crowd and his teammates loudly celebrated.
"We are a team, you need to have faith," Mayer said amid tears in his post-match interview.
The decisive match between Federico Delbonis and Thomaz Bellucci was suspended because of darkness after Delbonis won the first set 6-3 and was set to resume Monday.
In Vancouver, Canada, Vasek Pospisil beat Go Soeda 7-5, 6-3, 6-4 in the decisive singles match to give the hosts a 3-2 victory and set up a quarterfinal against Belgium.
Japan had tied the score at 2-all with fourth-ranked Kei Nishikori's 3-6, 6-3, 6-4, 2-6, 6-4 victory over sixth-ranked Milos Raonic in the first match Sunday.
Australia also advanced to the quarterfinals with a 3-2 win over the Czech Republic. Bernard Tomic won the decider over Lukas Rosol 7-6 (4), 6-3, 7-6 (5). The Australians will next play Kazakhstan, a 3-2 winner over Italy.