Yamashita was parachuted into the line-up of SARD's #39 GR Supra alongside Yuichi Nakayama when it became clear that Japan's entry restrictions for foreigners would prevent Kovalainen from being present for the first round of the delayed 2020 season.
Having got his first taste of the Supra in the pre-season test at Fuji the previous month, Yamashita helped the team to a fifth-place finish, as Toyota locked out the top five spots in the first race outing for its all-new GT500 contender.
Kenta Yamashita（#39 DENSO KOBELCO SARD GR Supra）
Yamashita took over the #39 Supra for the second half of the Fuji race, running seventh, and in the closing laps he passed the struggling Hondas of Tomoki Nojiri and Tadasuke Makino to come fifth behind the two TOM'S Toyotas and the Cerumo-run Supras.
"When I saw Toyota lock out the first five positions, I thought the car is already very complete," Yamashita told Motorsport.com of his first race since the Austin FIA World Endurance Championship round back in February.
"Even though I drove the car in the test, I quickly had the potential to set a good time so I felt like we could compete for a top position in the race.
"I knew the NSX-GT would be fast over a lap, but since the days of the LC500 the Lexus has always been good over a race distance, so [this time as well] I thought I could fight."
It's understood Yamashita will not be on call for SARD for the second round of the season at Fuji on August 8-9 as he heads to Spa to resume his WEC campaign for LMP2 squad High Class Racing, and he almost certainly won't be available for the third round at Suzuka either.
Yamashita added of his Fuji experience: "It was fun. It was great to feel the excitement and thrill of fighting in a close battle. I was able to get back my racing instincts too.
"I realised I was able to race properly even though I had a long time [with no racing], and my confidence grew."
#39 DENSO KOBELCO SARD GR Supra