Sasahara, whose deal to join Mugen in a second car was only announced days before the start of pre-season testing, went quickest of all in the 12-car Q2 pole shootout with a best time of 1m21.404s.
That was enough to beat Team Goh rookie Sato, who had topped his Q1 segment, by 0.264s.
Team Impul's Ryo Hirakawa qualified best of the Toyota runners in third behind an all-Honda front row ahead of practice pacesetter Ritomo Miyata (TOM'S) and Sacha Fenestraz (Kondo Racing), who had likewise topped his Q1 group but went marginally slower in Q2.
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Reigning champion Tomoki Nojiri was only sixth in the second Mugen car, 0.525s off the pace of teammate Sasahara.
Tadasuke Makino (Dandelion), Yuhi Sekiguchi (Impul), Giuliano Alesi (TOM'S) and Hiroki Otsu (Dandelion) made up the rest of the top 10.
Sho Tsuboi, who had been one of the favourites based on his pre-season test form, was only 10th-fastest of the 12 Q2 runners and had his best time erased for exceeding track limits.
That dropped the Inging driver to 12th on the grid behind Otsu and rookie Atsushi Miyake (Goh).
Kenta Yamashita (Kondo) was the surprise elimination of Q1, managing only the eighth-best time to put himself 15th on the grid, one place behind B-Max Racing's Nobuharu Matsushita.
Naoki Yamamoto's struggles with Nakajima Racing continued as he qualified down in 17th with the second-slowest time in his Q1 group, but his teammate Toshiki Oyu fared even worse as he ended up 21st and last.
Saturday's 41-lap race begins at 2.30pm local time (GMT +9).
Fuji Super Formula - Q2 results:
| Cla | # | Driver | Team | Time | Gap |
|---|---|---|---|---|---|
| 1 | 15 |  Ukyo Sasahara | Team Mugen | 1'21.404 | |
| 2 | 53 | Ren Sato | Team Goh | 1'21.668 | 0.264 |
| 3 | 20 | Ryo Hirakawa | Team Impul | 1'21.708 | 0.304 |
| 4 | 37 | Ritomo Miyata | Kuo VANTELIN TEAM TOM’S | 1'21.727 | 0.323 |
| 5 | 4 |  Sacha Fenestraz | Kondo Racing | 1'21.774 | 0.370 |
| 6 | 1 | Tomoki Nojiri | Team Mugen | 1'21.929 | 0.525 |
| 7 | 5 | Tadasuke Makino | Dandelion Racing | 1'21.974 | 0.570 |
| 8 | 19 | Yuhi Sekiguchi | Team Impul | 1'22.107 | 0.703 |
| 9 | 36 | Giuliano Alesi | Kuo VANTELIN TEAM TOM’S | 1'22.120 | 0.716 |
| 10 | 38 | Sho Tsuboi | P.MU/CERUMO･INGING | 1'22.140 | 0.736 |
| 11 | 6 | Hiroki Otsu | Dandelion Racing | 1'22.158 | 0.754 |
| 12 | 55 | Atsushi Miyake | Team Goh | 1'22.210 | 0.806 |
| View full results | |||||
