Hirakawa was chosen to replace Kazuki Nakajima aboard the #8 Toyota GR010 Hybrid this season off the back of his successes in both Super Formula and SUPER GT in recent seasons, joining Sebastien Buemi and Brendon Hartley.
The trio went on to take victory in June's double-points Le Mans round and at Fuji in September, with second place in this month's Bahrain finale behind the sister #7 car delivering Toyota its fourth straight WEC crown.
Reflecting on his success this year, Hirakawa said that his early struggles to get to grips with the GR010 Hybrid, which is considerably heavier and with less downforce than the cars he was used to racing in Japan, had sewn doubts in his mind over whether he could deliver.
"I didn’t drive well in the first couple of tests, I was so nervous, feeling the pressure," the 28-year-old told Autosport.
"But the team and my team-mates, not only car #8 but also car #7, helped me a lot. I caught up quite quickly thanks to them.
"When I think back to a year ago, when they first announced I was joining the team, I wasn’t sure if I could do it. I didn’t have confidence.
"This year we somehow achieved our goals, to win Le Mans and the championship. But a year ago I couldn’t believe that was possible."
Hirakawa's opportunity to join the Toyota WEC line-up came five years after he was rejected for a place in the marque's third LMP1 car at Le Mans in favour of compatriot Yuji Kunimoto in 2017.
He rebounded from that disappointment by winning that year's SUPER GT title alongside Nick Cassidy, with an especially impressive 2020 season opening the door for him to return to the Cologne-based outfit.
Asked about the significance of winning the title after what happened in 2017, Hirakawa replied: "The car is different from a few years ago, but it’s still similar, and that was a concern - that I couldn’t drive well with that car [TS050 Hybrid] and with this team. That’s why I was so nervous.
"Also the team, and especially the drivers, are so high-level, and to match that is not easy. Even at Le Mans, I needed to push as much as I could to match them, and to race them was a nice experience, to push myself to a higher level."
Speaking in Bahrain, Toyota Gazoo Racing Europe technical director Pascal Vasselon described Hirakawa's performances at Le Mans and Fuji as "really, really good," pointing out the Japanese driver was "always within one tenth" of the best average laptimes at La Sarthe.
Hartley was also effusive in his praise of Hirakawa in the wake of the #8 crew's title triumph over Alpine, with which they had gone into the Bahrain weekend equal on points.
"I think a really crucial moment for me was Le Mans, the step Ryo took at Le Mans under the pressure," said the New Zealander. "He had no sleep on the Friday he was nervous, he performed amazingly. That was a big moment in our championship.
“To then also to go to Fuji in our home race, Ryo’s home race, he was the teacher there, teaching us where the grip was.
“As a crew, I really felt we delivered a near-perfect race in Fuji and I think that gave us confidence as a car crew that we can challenge in the next years.”
