Sekiguchi, who has moved to SARD after winning last year's title for TOM'S alongside Sho Tsuboi, set the day's fastest time of 1m28.276s to give Toyota a clean sweep of top times in official pre-season testing.
However, Tsuboi's Saturday benchmark of 1m27.972s in the #36 TOM'S car stood as the best time across the two-day test.
The day started with poor weather, with the track staying wet throughout the two-hour morning session, which like Saturday's rain-affected afternoon session was topped by Nissan's two NISMO-run Zs.
Katsumasa Chiyo set the morning's best time of 1m36.895s in the #3 NDDP Racing-branded car, around a tenth up on the #23 machine of Ronnie Quintarelli.
The afternoon session - extended to three hours after Saturday's bad weather - likewise began in the wet, but as conditions improved Chiyo's morning benchmark was quickly surpassed.
At around the 40-minute mark the teams switched over to slick tyres, with Rookie Racing man Kenta Yamashita the first to break the 1m30s barrier after a little over an hour had elapsed.
With an hour left on the clock, a red flag was thrown to collect a piece of debris on the approach to Turn 1, after which Tomoki Nojiri moved to the head of the leaderboard with a 1m28.483s in the ARTA Honda.
That was then surpassed by Sekiguchi, who outpaced Nojiri to the tune of 0.207s at the wheel of the SARD GR Supra he shares with Yuichi Nakayama.
Third-fastest was the #17 Real Racing Honda of Koudai Tsukakoshi, less than a tenth back from Nojiri, while Nissan's top representative was the #3 car of Mitsunori Takaboshi on a 1m28.655s.
Naoki Yamamoto made it three Honda NSX-GTs in the top five, 0.004s slower than Takaboshi, while Yamashita ended up sixth in the Rookie Toyota, another thousandth behind.
The #36 TOM'S Toyota that had topped all three official test days prior to Sunday was only 13th-fastest with Giuliano Alesi at the wheel while the track was rapidly improving. Tsuboi took over for the closing stages for a long run.
GT300: Cozzolino fastest for Ferrari
In the GT300 class, Kei Cozzolino set the fastest time of the test aboard the Pacific CarGuy Racing Ferrari 488 GT3 Evo.
Cozzolino's best time of 1m36.142s narrowly surpassed the best time from Saturday set by the Team UpGarage Honda NSX GT3, which continued its strong form with the second-fastest time on Sunday.
Honda junior driver and SUPER GT novice Kakunoshin Ota left it late to post a best time of 1m36.543s, around three tenths slower than teammate Takashi Kobayashi had managed on Saturday.
JLOC's pair of Lamborghini Huracan GT3 Evos also continued their strong form, Yuya Motojima going third-quickest in the #88 car ahead of Natsu Sakaguchi in the #87 machine. Masataka Yanagida rounded out the top five at the wheel of Drago Corse's new Nissan GT-R NISMO GT3.
Causing a red flag in the morning session was the #48 NILZZ Nissan, which suffered a crash exiting Turn 2 with gentleman driver Joe Shindo at the wheel, also preventing the team from running in the afternoon.
Gainer's #10 Nissan meanwhile did not participate at all following its crash on Saturday.
Fuji testing times (GT500 class):
| Pos. | No. | Car/Team | Drivers | Morning | Afternoon |
|---|---|---|---|---|---|
| 1 | 39 | SARD Toyota |
| 1'38.227 | 1'28.276 |
| 2 | 8 | ARTA Honda |
| 1'37.613 | 1'28.483 |
| 3 | 17 | Real Honda |
| 1'39.234 | 1'28.572 |
| 4 | 3 | NDDP Nissan |
| 1'36.895 | 1'28.655 |
| 5 | 100 | Kunimitsu Honda |
| 1'38.742 | 1'28.659 |
| 6 | 14 | Rookie Toyota |
| 1'37.639 | 1'28.660 |
| 7 | 38 | Cerumo Toyota |
| 1'38.619 | 1'28.713 |
| 8 | 12 | Impul Nissan |
| 1'37.651 | 1'28.763 |
| 9 | 37 | TOM'S Toyota |
| 1'38.760 | 1'29.064 |
| 10 | 24 | Kondo Nissan |
| 1'38.353 | 1'29.070 |
| 11 | 19 | Bandoh Toyota |
| 1'37.958 | 1'29.086 |
| 12 | 16 | Mugen Honda |
| 1'40.741 | 1'29.180 |
| 13 | 36 | TOM'S Toyota |
| 1'38.100 | 1'29.390 |
| 14 | 64 | Nakajima Honda |
| 1'40.024 | 1'29.711 |
| 15 | 23 | NISMO Nissan |
| 1'36.989 | 1'29.717 |
Yuhi Sekiguchi
Bertrand Baguette
Sacha Fenestraz
Ronnie Quintarelli
