David Goffin had mercilessly swept aside close friend Lucas Pouille to give outsiders Belgium the first point at the Stade Pierre-Mauroy.
But world number 15 Tsonga steadied home nerves in the second rubber of the opening day with a 6-3, 6-2, 6-1 success over 76th-ranked Darcis to boost France's hopes of claiming the trophy for the first time since 2001.
France's top player, a beaten finalist against Switzerland at this same venue in 2014, took 1 hour 46 minutes to despatch the Belgian number two in the pair's first ever encounter.
Tsonga's power denied Darcis any chance of a comeback in the first set and it was plain sailing for the 32-year-old from then on.
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"After the first match my job was to get my team back to 1-1, no matter what the score was or how I did it.
"Whether it's us or the Belgians our mission is to bring home the salad bowl (trophy), that's the only thing that counts. If I could push the ball on with my teeth, I'd do it!"
"I came up against someone who was stronger than me today. The idea was to impose my style of play and stop him doing the same, it didn't come off."
Tsonga's success left the 2017 final finely poised ahead of Saturday's doubles with France captain Yannick Noah still hesitating whether to stick with his proposed pairing of Richard Gasquet and Pierre-Hugues Herbert, or throw Tsonga into the mix.
Noah suggested the 1-1 scoreline was "logical".
"It was similar with Jo. We knew Darcis was a Davis Cup player. He loves to dig in then change a gear to win. But Jo didn't give him the opportunity to do that.
"He (Tsonga) was very solid, he played a very good match. I was delighted. So 1-1 is logical."
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Goffin, the world number seven, won 7-5, 6-3, 6-1 in front of a crowd of close to 26,000 fans, many making the short trip from across the border with Belgium.
This was the 26-year-old's first win over 18th-ranked Pouille in their fourth meeting.
Goffin got the better of his French foe's strong baseline play, making the telling break in the 11th game to serve out to love to take the opening set.
He broke Pouille's serve at the start of the second, going on to break again to take the set 6-3, and race to a 3-0 lead in the third.
"I've pushed the salad bowl a little towards the border, only a little because it's heavy," said Goffin, referring to the trophy.
"I'm thrilled to have set the team off like that."
Pouille was magnanimous in defeat.
"On that form he should have won the Masters. There's not much to say, he's playing the tennis of his life."
Noah was keeping his cards close to his chest meanwhile on his pairing for Saturday's doubles saying he may leave a decision "until the last moment".
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