There were doubts about the world number five's fitness heading into the tournament after he was forced to pull out of the Halle semi-finals nine days ago due to a calf injury.
The calf in question was lightly strapped for much of Nishikori's 6-3, 6-7(4), 6-2, 3-6, 6-3 victory, with the 25-year-old forced to have it re-taped by the trainer in the middle of the fifth set.
Speaking after the match, Nishikori conceded that the contest had been a battle, but that it was one he had expected and didn't expect to feel any after effects from.
"It's a little bit sore from last week but it's getting better and it wasn't easy to play for three hours but it should be OK for the next match," he said.
"I knew it was going to be a tough one because we played long five sets last year here. I knew he's good on grass," the Japan ace added.
"Mentally, I was ready but there was many ups and downs. My serve, and maybe my concentration wasn't there some moments."
However, while not completely happy with his performance, Nishikori believes that there are positives to take from the match as he prepares to face Colombian Santiago Giraldo in the second round.
"Finish very strong in the end," he said.
"Sure I need more matches to get more experience. And also to win the matches, it's important for my confidence. I'm sure that I'm playing good on grass now."