The win moved the Royals above their opponents and they now sit top of the Championship table, knowing that four points from their final three matches will guarantee them a return to the top flight.
McDermott told Sky Sports News: "That was a big performance. They're a good side Southampton, who played really well, and we had to stay in the game, which we did.
"I don't know what's going to happen. People say 'if this happens'. If is a big word and the only thing that concerns us is now is we play Nottingham Forest in front of our own fans.
"It's going to be great to go home and to play the next two games at home because we've had a couple of really difficult games away from home and we've managed to get six points. I'm so proud of the players."
Le Fondre has now scored nine goals this season for the Berkshire club, after returning from a brief loan spell at Rotherham United in August.