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.
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.