United let slip their advantage to draw 3-3 against the South Coast club having managed to let three points escape them at Leicester on Boxing Day two days earlier.
The result left Grayson frustrated after his side had looked like securing a comfortable victory, but he was still happy to see his team extend their unbeaten run to 11 matches.
United sit fourth after the draw and Grayson told the club's official website: "It was a strange one. We had a two-goal lead again and had opportunities to see it through. Portsmouth had plenty of possession and we knew they would have opportunities.
"(David) Nugent got a great goal for them, but we switched off having just made it 3-1 and that was a pivotal moment. If we'd gone five or 10 minutes at 3-1 we would have won. When you're in this division teams will get phases where they get opportunities and you have to have the nous to see it through.
"But there's plenty of positives there. We're still unbeaten, I was happy with large parts of the performance, particularly the way our wide players performed and the way we passed it around. It's just frustrating that we didn't see it out."
Source: PA