They will travel to Scunthorpe or Tranmere, who battle it out at Glanford Park for the final play-off spot on Saturday, in the first leg before having home advantage in the deciding second leg.
And after watching his side shutout the Cobblers, Di Matteo said his side will be in confident form for the play-offs.
"We were rock-solid. It was a very good performance from the back five, but I think the whole team worked very hard tonight," he said.
"It's exciting to have a team like this. We know our home form could have been better, but we also know the reasons behind it and we'll try to address that in the summer.
"If we can finish third I think it will have been a great season for us. You have to consider that we have the smallest squad in League One.
"We've scored the most goals in all the professional leagues, we play at times very exciting football and if we finish third in our first season in League One - with very important players like Luke Chadwick and Jason Puncheon having only played 50 per cent of the season - that says a lot about the other players."