Lille played without fear and pressed relentlessly and prevailed courtesy of a Jonathan David penalty in first-half stoppage time to wrap up a memorable win and hand Carlo Ancelotti's team their first defeat in all competitions since January.
"What this team did is incredible. We try to put things in place but you need the players to believe in the plan for this kind of upset to happen," Genesio said.
"You have to do everything perfectly, you need a keeper who makes the decisive saves, a striker who scores and a bit of luck."
Keeper Lucas Chevalier certainly did his job, making a handful of spectacular saves when Real finally applied some proper pressure.
Lille managed to keep it together after a great first half, during which they had the upper hand against a timid Real side.
"We played a very good first half, we played together, it's a great night for everyone," Genesio added, explaining he and his staff had studied Real's 1-1 draw with Atletico Madrid in LaLiga last weekend.
"We used this game to prepare. We saw that Real like to cut inside so we put intensity in that sector even if we had to abandon possession sometimes," he said.
"Then it was all about pressing when it mattered, everything worked perfectly, it was an ideal scenario even if we suffered a lot in the last 25-30 minutes but Lucas made the saves we needed."
His Real counterpart Carlo Ancelotti hoped the team would learn from the defeat after they had gone unbeaten for 36 games in all competitions.
"It seemed like we were improving and today we took a step back," the Italian said.
"The loss against Atletico in January helped us to change. Hopefully the same will happen now."