Johnson scored his first goal in last weekend's defeat at Everton in what was perhaps the England midfielder's best individual display for the Black Cats.
The England international admits he was not at his best in the early weeks of his spell at the Stadium of Light after an injury in pre-season at Manchester City and he now hopes he has turned the corner.
"I felt good. It's been coming," said Johnson. "The injury set me back a bit when I signed and I sort of lost the whole pre-season I did with City through one injury.
"I think my fitness is definitely getting there now and hopefully my form will come.
"It's probably the best I have played on Saturday since I signed."
Sunderland take on Fulham on Sunday, having won just one of their last 18 league games and Johnson is confident they will soon get back to winning ways.
"It's only a matter of time. It [Everton] was definitely the best we have played for a few weeks and we can look at the positives," added Johnson.
"Hopefully there are going to be a few wins in the next couple of weeks."