Harlequins and Racing 92 will both go into the final round of pool games in the Heineken Champions Cup hoping to scrape into the knockout stage.
The Top 14 side ended a run of five games without a win as they pipped Quins by a point thanks to a 77th-minute penalty from Nolann Le Garrec that came from the restart moments after Finn Russell had picked up their third yellow card in as many minutes.
Russell, who had a great battle with the fit-again Marcus Smith at outside half, was sent to the sin-bin for a deliberate knock-on. Irish referee Andrew Brace also awarded a penalty try that pushed Quins into the lead for the first time in the game at 29-27.
Down to 12 men - earlier Quins had been reduced to 13 when Aaron Morris and Alex Dombrandt were yellow-carded - Racing somehow managed to win a penalty from the restart to give Le Garrec his shot at glory to earn a 30-29 victory.
“It was a massive win for us. To put it into context we came into the game after four losses and a draw, so it was nice to get another 'W' on our resume,” said former Harlequins centre, Francis Saili. “It was a tough match out there and I'm bloody tired, although it was bloody nice to score against my old club.
“We knew it was going to be quick and we knew they were going to throw everything at us, given the brand of rugby they play. They are similar to us in they just throw the ball around.
“It turned into a bit of an arm wrestle going back and forth, especially with the number of yellow cards we had. I'm just glad we came out on top.”
While Quins head home to the Twickenham Stoop to host South African side the Sharks, Racing go to Dublin to face unbeaten Leinster. Both teams know they need points to stay in the tournament.
“This win gives us a foot in the door, although we know getting another foot in the door is going to be an uphill battle. It's not going to be an easy task for us to go over to Leinster,” added Saili.
“They are really hot at the moment. We will analyse them, but we need to look at ourselves and see what positives we can do so we can throw everything at them.”
