Teenage forward Jed Wallace bagged his third goal in three matches after three minutes to put Pompey ahead but Keith Keane's stunning volley after 77 minutes levelled for the Lilywhites.
Portsmouth rode their luck at times - Jack King headed against the bar and Joe Garner hit a post before the hosts eventually equalised - and while North End have lost just once in eight games, they have drawn four of those.
Grayson said: "It was a game where we created some good chances but we've dropped two points. We've hit the bar and the post.
"We're disappointed with the goal we conceded but full credit to the players, they stuck to it.
"We had enough chances to win the game and that's disappointing but if you don't win it you make sure you keep fighting and scrapping to make sure you get something out of it.
"It was a great goal. I thought he did well when he went into midfield and Keith has been very good for us since I've been here.
"It needed to be something like that goal as their keeper had made some good saves. Once we got the goal it looked like we were going to kick on and get that winner, but it wasn't to be.
"They kept plugging away which is the good thing and they didn't let the early goal affect them and they carried out the instructions I wanted them to do."