Toulon’s hopes of exerting a vice-like grip on Pool Three were extinguished after Welsh fly-half Williams booted six penalties and converted lock Brad Thorn’s 19th-minute try.
Leicester ultimately prevailed 25-21, but they were made to sweat following an impressive Toulon revival from 13-0 adrift as wings Bryan Habana and Drew Mitchell claimed touchdowns, while fly-half Nicolas Sanchez landed three penalties and a conversion.
While the reigning European champions still top their group, courtesy of a losing bonus point, Leicester have moved themselves firmly into the shake-up prior to next Saturday’s return fixture at Stade Felix Mayol.
Toulouse claimed their 100th victory in European rugby and put themselves in pole position to qualify for the quarter-finals with a 19-11 victory over Glasgow Warriors.
The Scottish side will have to win the reverse Pool Four fixture at Scotstoun next week to keep alive their hopes of getting through to the knockout stages for the first time.
Yellow cards for Finn Russell and Leone Nakawara cost the Warriors as Duncan Weir missed a late conversion which would have given them a losing bonus point at Stade Ernest Wallon.
Glasgow director of rugby Gregor Townsend admitted indiscipline cost his side from securing a famous victory.
The four-time European champions from the south of France scored 16 of their 19 points in the 10-minute periods whenRussell and Nakawara were in the sin-bin.
Townsend said: “The two yellow cards put us under a lot of pressure and gave a team like Toulouse, who were off-loading all over the place, a chance to find their gap.
“But their defence was excellent so we’ll have to look at ways to break that down better.
“The penalty count was 14 to five and when you come away from home playing against one of the best teams in Europe with two yellow cards it is very tough to win.
“We should have put more points on the board in the first 30 minutes when we were dominating the game.”
Meanwhile, Wasps director of rugby Dai Young has warned his side to guard against complacency after their bonus-point 32-17 European Champions Cup victory over Castres Olympique, insisting that the job is “only half done”.
A penalty try and touch downs from Rob Miller, Nathan Hughes and Sailosi Tagicakibau were enough for Wasps to win the battle of Pool Two’s bottom sides, while Andy Goode added 12 from the kicking tee.
Mathieu Bonello and Remi Lamerat crossed over for the hosts in reply but it was not enough to stop Wasps, who can now target the sides above them in the group.
“We knew coming into these two games that we would have to win them to go into the last two games with any realistic possibility of qualifying,” Young said. “Obviously, we have got to back this up next week.”
He added: “We have seen today that Castres have some great individuals, so next week’s match isn’t a foregone conclusion.