Surprise packages Lille, nicknamed the 'Mastiffs', had not won against any of the teams in the tight top five this season and they were heading for a draw until substitute Frau poked in Emerson's low cross.
Marseille's defeat sent them down to fourth and propelled Olympique Lyon up to third, four points behind Lille, after Lisandro Lopez scored a hat-trick in a 5-0 demolition of bottom side AC Arles-Avignon.
Second-placed Stades Rennes, who won 1-0 at Montpellier on Saturday and host Marseille on Friday, are level on points with Lille but Rudi Garcia's side have a better goal difference which could be key with just 12 games left.
Lille winger Eden Hazard, who signed a contract extension this week to scare off Europe's elite, netted on 10 minutes when he let fly from 25 metres and the ball caught the wind inside the open Stade Velodrome and flew into the top corner.
Marseille were subdued in the first half, partly because of Lille's confident play which could have heralded a second goal had Gervinho not missed a gilt-edged chance.
"It's true that it was Arles-Avignon but they'd just had three draws. We approached the match like we should have and made a great start," coach Claude Puel told reporters.
"I'm very confident," Lisandro said. "I won't leave Lyon til we have won this title. it will be this season or next."
Sunday's three games featured three times the amount of goals [four] scored in seven matches on Saturday when fifth-placed title hopefuls Paris Saint-Germain lost 1-0 at lowly Auxerre.
