Roger Federer slumped to his first Davis Cup defeat in nine years on Friday when he was shocked 4-6, 6-3, 7-6 (7/4), 6-2 by John Isner as the United States opened up a 2-0 lead over Switzerland.
World number three Federer, playing a World Group first round tie for the first time in eight years, saw his 15-match winning streak in the competition ended by the giant Isner on the indoor clay courts at Fribourg.
Isner secured victory with three return winners as the 32-time Davis Cup champions moved closer to a quarter-final spot.
Federer insisted that he and Stanislas Wawrinka, the Olympic doubles champions, can still pull the tie around by winning Saturday's doubles against Mike Bryan and Ryan Harrison.
"I played a good match considering the altitude and the fact that it was my first match on clay since the French Open," said Federer.
"It's a tough loss, but we are not down and out yet. Stan and I will pick ourselves up just like we did at the Olympics."
Isner described the win as the best of his career and gave credit to captain Jim Courier.
"The way I played today is the way I need to play in all my matches. I owe it to Jim Courier -- he was on at me to hit all my shots. It's the win of my life," said Isner, who unleashed 85 winners.
Courier said Isner got his tactics just right against the 16-time Grand Slam title winner whose last Davis Cup singles loss came in 2003 when he was defeated by Australia's Lleyton Hewitt in Melbourne.
"The mission was to go out there and not allow an artist a canvas to work with," said Courier.
Earlier, Mardy Fish had defeated Wawrinka 6-2, 4-6, 4-6, 6-1, 9-7 in the opening rubber.
"There's something special about the Davis Cup. You just never give up," said Fish.
Defending champions Spain, who are without Rafael Nadal, were comfortably placed at 2-0 ahead of Kazakhstan in Oviedo.
Veteran Juan Carlos Ferrero clawed out a 6-1, 4-6, 7-6 (7/2), 4-6, 6-4 win over Mikhail Kukushkin before Nicolas Almagro defeated Andrey Golubev 6-3, 4-6, 6-1, 6-1.
Nadal is skipping the tournament to concentrate on the Grand Slams as well as the defence of his Olympic title.
World number one Novak Djokovic is also absent from 2010 champions Serbia's tie in Nis against Sweden.
But he wasn't missed as Janko Tipsarevic beat Filip Prpic 6-3, 6-3, 6-4 and Viktor Troicki saw off Michael Ryderstedt 6-4, 6-3, 5-7, 6-3 for a 2-0 lead.
World number nine Tipsarevic had no problems against an opponent ranked 1,417 places below him while Ryderstedt, who replaced long-term illness victim Robin Soderling, at least took a set off Troicki.
In Bamberg, last year's beaten finalists Argentina put a foot in the quarter-finals when they opened up a 2-0 lead against Germany.
Juan Monaco beat Philipp Petzschner 6-3, 6-3, 6-3 before David Nalbandian battled back to defeat Florian Mayer 2-6, 6-0, 6-1, 7-6 (7/5).
In Hyogo, Ivo Karlovic fired 18 aces to claim a straightforward 6-4, 6-4, 6-3 victory over Australian Open quarter-finalist Kei Nishikori as Croatia pulled level at 1-1 with Japan who are back in the World Group for the first time in 26 years.
Japanese number two Go Soeda had earlier stormed back from two sets down to beat Ivan Dodig 6-7 (3/7), 3-6, 6-4, 6-3, 7-5.
At Wiener Neustadt, Austria were 2-0 ahead of Russia with Jurgen Melzer claiming a 6-2, 6-7 (3/7), 6-4, 3-6, 6-1 win over Igor Kunitsyn before Andreas Haider-Maurer beat Alex Bogomolov 6-1, 6-4, 6-7 (1/7), 6-2.
The Czech Republic were 2-0 ahead against Italy in Ostrava after 33-year-old Radek Stepanek defeated Andreas Seppi 4-6, 6-4, 6-3, 3-6, 6-3 and Tomas Berdych had seen off Simone Bolelli 6-3, 4-6, 7-5, 6-2.
In Vancouver, Canada and France ended the opening day level at 1-1. Jo-Wilfried Tsonga gave the first point to France with a 6-1, 6-3, 6-3 victory over Vasek Pospisil. Rising Canadian star Milos Raonic then knotted the tie with a 6-2, 6-4, 7-5 victory over Julien Benneteau, who was a late substitute in singles for Gael Monfils, who is nursing a knee injury.