After being substituted in the 41st minute due to a muscle injury, Sissoko joined the rest of the Spurs players on the bench but stormed down the tunnel believing Raheem Sterling last minute goal had knocked his side out of the competition.
The strike was subsequently ruled out by VAR, though, and Spurs progressed on away goals despite losing a thrilling game 4-3.
French midfielder Sissoko has now revealed that he was unaware the goal had been chalked off, telling LEquipe of his joy after finding out Spurs had progressed.
"I was on the bench when I saw the goal that made it 5-3," Sissoko began. "I was so upset that I returned directly to the dressing room, I was alone, no one accompanied me, no TV screen was broadcasting the game.
"In my head we were eliminated, then one of the staff members entered the dressing room and told me ;incredible, we did it!. I said how did we do it? At that moment, he told me that the goal had been disallowed."
Simply, incredible.⏱️ 04 1-0 Sterling (1-1)⏱️ 07 ⚪️ 1-1 Son (1-2)⏱️ 10 ⚪️ 1-2 Son (1-3)⏱️ 11 2-2 B.Silva (2-3)⏱️ 21 3-2 Sterling (3-3)⏱️ 59 4-2 Aguero (4-3)⏱️ 73 ⚪️ 4-3 Llorente (4-4) #UCL ⚪️ #COYS pic.twitter.com/A9UazdAJoc
— Tottenham Hotspur (@SpursOfficial) April 17, 2019
Once told the stunning news, Sissoko admitted to having a rush of adrenaline, explaining that the pain from his injury subsided in that moment.
He added: "I put on a t-shirt, I forgot about my injury and I ran outside to join the others, not to miss the celebrations for such a historic moment.
"It was an incredible, emotional shock."
Sissoko has enjoyed his most consistent season yet as a Spurs player, playing 39 games and providing four assist in all competitions.
Spurs will play Ajax, in their first ever Champions League semi-final, on 30 Aprill, but will first face Manchester City again in the Premier Leagues early Saturday kick-off.