Veteran Turkish striker Burak Yilmaz opened the scoring on the hour mark.
The 35-year-old collected Renato Sanches' through ball and slotted past Metz goalkeeper Alexandre Oukidja for his 10th goal in 22 league games.
Yilmaz’s compatriot, Mehmet Zeki Celik, secured the victory in the 89th minute.
Lille goalkeeper Mike Maignan provided the platform for the success.
Mike Maignan est une formidable raison d’aimer le football ❤️
— LOSC (@losclive) April 9, 2021
Le seul, l’unique, le Numéro Uno, le Magic Eagle : @mmseize 🦅#FCMLOSC pic.twitter.com/UNQBe7DnBg
The 25-year-old France international stretched to his right to tip away Aaron Leya Iseka's spot kick in the 17th minute. He also produced a string of saves before Yilmaz's strike.
Lille boss Christophe Galtier highlighted the need for application and resilience during the run-in to a possible first top flight crown since a league and cup double in 2011.
His side heeded his advice. They showed grit and determination at the Stade Saint Symphorien to consolidate last Saturday's 1-0 win over Paris Saint-Germain at the Parc des Princes.
"The title? It's still a long way off," Galtier told Canal Plus. "And after the games on Saturday and Sunday, it will still be far off. But after tonight's success, we are guaranteed to be playing in Europe next season.
"Even though it's close, we can fight for something exceptional. I want the players to continue like they played tonight in the last six games. I want determination and motivation.
"But we mustn't let the stakes make us nervous."
Following Friday night's triumph, Lille boast 69 points after 32 games. However, PSG, champions in seven of the past eight seasons, can cut the lead to three points if they win at Strasbourg on Saturday afternoon.
Monaco and Lyon, third and fourth respectively, can maintain their interest in the title race with victories on Sunday against Dijon and Angers.
