Midfielder Tousart netted in the 31st minute to give Lyon a lead to defend when they head to Turin for the return on March 17.
The goal came with Juventus defender Matthijs de Ligt off receiving treatment, but it was just reward for an excellent first half by a Lyon side enduring a mediocre campaign.
They then survived late Juventus pressure to claim their finest European result since defeating Manchester City in last season’s group stage.
“It was a top-class first-half performance from us. It was much harder in the second half, but I was happy with how we hung in there,” said Lyon coach Rudi Garcia.
“I said to the boys that when you’re at home in the first leg it’s very important not to concede, and now we need to go and score there. We must remain humble, it’s only half-time.”
Maurizio Sarri’s side have genuine ambitions of winning a trophy that has evaded Juventus for almost a quarter of a century now, but they could not claim a precious away goal here, with Paulo Dybala seeing a late effort disallowed.
“In the first half it was not the Juve that we know. We didn’t get going in attack. I am struggling to understand certain things,” said Sarri, who thought his team could have had two penalties.
“We didn’t do enough given this was a Champions League match but fortunately we still have 90 minutes to turn it around.”
The match went ahead before a full house at the Groupama Stadium, including around 3,000 Juventus supporters allowed to attend despite calls from some French politicians to block their access amid fears about the spread of the coronavirus.
Those fans, and the Juve squad, would have crossed the Alps with confidence. The Serie A leaders came as overwhelming favourites against a Lyon team looking like possibly the weakest left in the competition.
The visitors’ attack was led by a Ronaldo in fine shape, with the 35-year-old having scored in all nine appearances in 2020 and netted 19 times since the beginning of December.
– Juve saved by woodwork –
He and his club — beaten in five Champions League finals since last lifting the trophy in 1996 — are in a hurry to succeed in this competition, but they did not play like contenders to go all the way.
Bidding to win a Champions League knockout tie for the first time in a decade, it was Lyon who quickly gained the upper hand, with new signing Bruno Guimaraes outstanding anchoring the midfield.
Karl Toko-Ekambi, another January signing, headed off the bar from Houssem Aouar’s corner midway through the first half. It was a warning to Juve of what was to come.
– De Ligt left bloodied –
Sarri’s team went temporarily down to 10 men after De Ligt was left stricken, blood streaming from a head wound, following a collision in Lyon’s area.
With the Dutchman off, Lyon pounced. Aouar burst into the box from the left and teed up Tousart to score from six yards.
With the heavily bandaged De Ligt back on, Lyon continued to threaten, Toko-Ekambi twice firing off target.
Juventus sent on Gonzalo Higuain and Aaron Ramsey in the second half and started to push Lyon back.
Nevertheless, they struggled to create chances, with Ronaldo rarely looking like adding to his recent tally and having a late penalty appeal rejected.
Higuain squandered a good chance before Dybala was denied by the offside flag, and the Turin giants will have to turn the tie around in three weeks.
“We believe in ourselves. With our fans right behind us, we can progress through to the next stages,” insisted Ramsey.
They can look to what happened at the same stage a year ago for inspiration — then, they needed to overturn a 2-0 deficit against Atletico Madrid, and a Ronaldo hat-trick did the job in the return at home.
