Teammate George Russell came seventh and Mercedes have fallen further behind in the Formula One constructors' standings - fourth overall on 79 points while third-placed McLaren, who also use Mercedes engines, have 154.
"We are in no-man’s land. There wasn’t much more for us to get today," said Hamilton, the sport's most successful driver of all time who is moving to Ferrari at the end of the season and was racing in Italy for the first time since that was announced.
"It is where we are and we have to do the best we can and that is the best we could do today."
Russell also referred to 'no-man's land' but said the once-dominant team were still "super-motivated".
"We have got to just keep pushing. The car was capable of P6 and P7 and that is where we finished."
Team boss Toto Wolff felt Mercedes had taken a small step forward.
"Our closest competitors have also done so recently, but we have closed the gap to the front a little. We still have a lot of work to do and, of course, we are all frustrated finishing P6 and P7," said the Austrian.
"There is more to come though, and it is all about making incremental gains."
