Roger Federer and Stan Wawrinka beat French doubles duo Julien Benneteau and Richard Gasquet to give Switzerland a 2-1 lead in the Davis Cup final.
Federer showed no signs of his recent back problem as the Swiss won 6-3 7-5 6-4 on the indoor clay court in Lille.
The 33-year-old, who lost his singles match to Gael Monfils on Friday, said: "Clearly it's a big relief. The medical staff, it's a big thanks to them."
There are two singles matches to play on Sunday, which will decide the final.
Federer is likely to take on Jo-Wilfried Tsonga and Wawrinka face Monfils as the Swiss seek to capture the first Davis Cup title in their history.

BBC tennis correspondent Russell Fuller
"Federer believes that a lack of clay court preparation, rather than an ailing back, undermined his chances on the opening day - and he certainly moved with great agility in the doubles.
"Wawrinka was again outstanding and they outclassed a French pair which had never played together in the Davis Cup before.
"Gael Monfils will be a formidable competitor for France on the final day, but his match with Wawrinka will be academic unless Jo-Wilfried Tsonga casts off his inhibitions of Friday and beats Federer in Sunday's opening match."
The respective world number one and four were in charge of Saturday's doubles match from the start, in front of a 27,000 crowd at the Stade Pierre Mauroy.
Wawrinka, 29, said: ""I am really happy with the way we were playing today. We were really aggressive, we knew what we had to do. I think we did a good job."
The Swiss duo secured the first break of the match on the Benneteau serve in the sixth game and that proved enough to take the opening set.
Their French opponents came out fighting in the second set but failed to convert any of five break points that came their way.
With the set seemingly destined for or a tie-break, Gasquet, who never managed to match his partner's impact on the match, failed to hold serve.
AP
France's Richard Gasquet struggled to find his best form alongside partner Julien Benneteau.
A Federer backhand winner secured the break to make it 6-5 and Wawrinka served out for a two sets to love lead.
The French pair attempted to rally in the final set but with the otherwise impressive Benneteau struggling on serve, Wawrinka converted a third break point opportunity in the fifth game of the set.
The Swiss never looked back, with Federer, who had been moving through the gears since midway through the second set, serving to love for the match.
"The coaching staff did a wonderful job of getting us ready today," he said. "Thanks for getting me back onto the court."
Swiss duo Marco Chiudinelli and Michael Lammer were originally slated to play the match, but Federer and Wawrinka were drafted in with so much at stake.

As it stands: France 1-2 Switzerland
21 Nov: Stan Wawrinka (Swi) beat Jo-Wilfried Tsonga (Fra) 6-1 3-6 6-3 6-2
21 Nov: Gael Monfils (Fra) beat Roger Federer (Swi) 6-1 6-4 6-3
22 Nov: Julien Benneteau & Richard Gasquet (Fra) v Roger Federer & Stan Wawrinka (Swi) 6-3 7-5 6-4
23 Nov: Jo-Wilfried Tsonga (Fra) v Roger Federer (Swi)
23 Nov: Gael Monfils (Fra) v Stan Wawrinka (Swi)
Switzerland now need to win just one of Sunday's reverse singles matches to claim the title, but Wawrinka is expecting a hard day.
"It's a tough, long weekend," he said.
"We are going to get ready in the best possible way for tomorrow. We are going to rest now and be ready - Roger and me - to play."
In Friday's opening singles, Wawrinka defeated Tsonga in four sets and Monfils brushed aside Federer, who was making his return after pulling out of Sunday's ATP World Tour final with a back problem, in three.