Switzerland's Roger Federer was encouraged by his fitness levels in Friday's one-sided Davis Cup final defeat by Frenchman Gael Monfils.
Federer was beaten 6-1 6-4 6-3 as the opening day finished all-square in front of a Davis Cup record 27,432 fans at the indoor clay court in Lille.
The 33-year-old, who pulled out of Sunday's ATP World Tour final, will now partner Stan Wawrinka in the doubles.
"I started to feel better as the match went on," said Federer.
"That's very encouraging, I must say. I would think that I'm going to get better as the weekend goes on."
Wawrinka beat Jo-Wilfried Tsonga 6-1 3-6 6-3 6-2 in the opening singles rubber to give Switzerland a 1-0 lead, but Monfils's win over Federer drew the tie level at 1-1.
Marco Chiudinelli and Michael Lammer had initially been named as Switzerland's doubles team, but Federer and Wawrinka will now face Richard Gasquet and Julien Benneteau.
|BBC tennis correspondent Russell Fuller|
|"With the support of a world-record crowd to feed off, and sensing an opportunity against an opponent physically below his best, Gael Monfils was magnificent from the start.|
|"Stan Wawrinka was equally impressive for Switzerland in the opening match of the day, despite the anxiety he felt at the extra responsibility on his shoulders."|
The best-of-five contest concludes with the reverse singles on Sunday.
France have not won the Davis Cup since 2001, while Switzerland have never won it.
The Swiss made the perfect start with Wawrinka winning 25 points at the net and sealing victory with a volley on his first match point.
The French hit back though, with Monfils winning the first point of his match against Federer with an ace, one of nine he sent down in a first set that raced by in just 31 minutes.
|As it stands: France 1-1 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)|
|23 Nov: Jo-Wilfried Tsonga (Fra) v Roger Federer (Swi)|
|23 Nov: Gael Monfils (Fra) v Stan Wawrinka (Swi)|
A break in the fifth game of the third set saw Monfils, who had never previously beaten Federer on clay, take control and he sealed victory with a backhand drive down the line at match point.
"That definitely is one of the top three matches in my life," Monfils said on the Davis Cup website.
"I was very nervous in the beginning. When Jo lost, it's an extra pressure because we need to get back on the road to win the title."
France have won 10 of their previous 12 Davis Cup ties against Switzerland and have won the title on 10 occasions.