The Frenchman had a dismal start to the season, failing to score in the opening eight races.
After that opening part of the campaign, Magnussen had 27 points and was battling for the ’best of the rest’.
Grosjean battled back in the second half of 2018, bringing the gap between the pair back to 19 points, and it would have been closer had he not been disqualified from the Monza race for a technical infringement.
“He scored a lot early in the year and I didn’t score anything,” Grosjean said.
“Then obviously there were the eight points of Monza that I’ve lost. It is not one or two, it is a big chunk, almost half of the gap.
“So, if you look from Germany to now, I think I have maybe had an upper hand.
“Early in the year he did a really good job. He was fifth in Bahrain, that’s 10 points, then than he kept going and I kept having either issues or just making mistakes and didn’t score points.
“Yes, the gap is big, [but] it was bigger initially, and I think I’ve done a great job, you know, coming back. Even in qualifying, because I was miles behind and I’ve done pretty well.”
Grosjean’s momentum in the second part of the year helped him beat his team-mate 10-9 in qualifying over the course of the season.
He admitted that he had to overcome a mental issue that had undermined his early season form.
“Of course, it was frustrating, it was annoying, it was hard,” he added.
“I was criticised a lot. But I knew that when things will go right, than everything would be settled, and then I will be back.
“There were a few mistakes, a few bad luck, all in together. Mistakes that I shouldn’t have done with my experience. But anyway, it’s always the way you react which is important, the way you come back.
“In sport in general there is no middle. It is either good or bad, but the middle is never really an option.”