# 2023 Keio Challenger – Men's singles

Christopher O'Connell was the defending champion but chose not to defend his title.
Yosuke Watanuki won the title after defeating Yuta Shimizu 7–6$^{(7–5)}$, 6–4 in the final.

## Seeds

* Yosuke Watanuki (champion)
* Jurij Rodionov (first round)
* Michael Mmoh (semifinals)
* James Duckworth (second round, withdrew)
* Maxime Cressy (withdrew)
* Luca Nardi (quarterfinals)
* Marc Polmans (second round)
* Leandro Riedi (second round)

## Draw
### Key

* Q = Qualifier
* WC = Wild card
* LL = Lucky loser
* Alt = Alternate
* SE = Special exempt
* PR = Protected ranking
* ITF = ITF entry
* JE = Junior exempt
* w/o = Walkover
* r = Retired
* d = Defaulted
* SR = Special ranking

### Finals

| Semifinals | Semifinals | Semifinals | Semifinals | Semifinals | Final | Final | Final | Final |
| 1 | Yosuke Watanuki | 6 | 6$^2$ | 6 | | | | |
| 3 | Michael Mmoh | 4 | 7$^7$ | 0 | | | | |
| 3 | Michael Mmoh | 4 | 7$^7$ | 0 | 1 | Yosuke Watanuki | 7$^7$ | 6 |
| | | | | | | Yuta Shimizu | 6$^5$ | 4 |
| | Yuta Shimizu | w/o | | | | Yuta Shimizu | 6$^5$ | 4 |
| Q | Yasutaka Uchiyama | | | | | | | |

### Top half

| First round | First round | First round | First round | First round | Second round | Second round | Second round | Second round | Second round | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Semifinals | Semifinals | Semifinals | Semifinals | Semifinals |
| 1 | Y Watanuki | 6 | 6 | | | | | | | | | | | | | | | | |
| | Y-h Hsu | 3 | 3 | | 1 | Y Watanuki | 6 | 6 | | | | | | | | | | | |
| Q | C Sinclair | 3 | 2 | | LL | M Erhard | 4 | 3 | | | | | | | | | | | |
| LL | Mathys Erhard | 6 | 6 | | | | | | | 1 | Y Watanuki | 3 | 6 | 6 | | | | | |
| Q | A Holmgren | 4 | 1 | | | | | | | LL | G Fonio | 6 | 2 | 1 | | | | | |
| LL | G Fonio | 6 | 6 | | LL | G Fonio | 4 | 7$^7$ | 6 | | | | | | | | | | |
| WC | R Noguchi | 2 | 2 | | 8 | L Riedi | 6 | 6$^4$ | 4 | | | | | | | | | | |
| 8 | L Riedi | 6 | 6 | | | | | | | | | | | | 1 | Y Watanuki | 6 | 6$^2$ | 6 |
| 3 | M Mmoh | 6 | 6 | | | | | | | | | | | | 3 | M Mmoh | 4 | 7$^7$ | 0 |
| | M Echargui | 1 | 2 | | 3 | M Mmoh | 6 | 5 | 7$^7$ | | | | | | | | | | |
| | G Blancaneaux | 6 | 6$^1$ | 6 | | G Blancaneaux | 4 | 7 | 6$^3$ | | | | | | | | | | |
| Q | O Jasika | 4 | 7$^7$ | 3 | | | | | | 3 | M Mmoh | 7$^8$ | 2 | 6 | | | | | |
| Q | K Ivanovski | 6 | 5 | 6 | | | | | | 6 | L Nardi | 6$^6$ | 6 | 4 | | | | | |
| | Z Bergs | 1 | 7 | 2 | Q | K Ivanovski | 2 | 4 | | | | | | | | | | | |
| LL | Y-s Chung | 2 | 4 | | 6 | L Nardi | 6 | 6 | | | | | | | | | | | |
| 6 | L Nardi | 6 | 6 | | | | | | | | | | | | | | | | |

### Bottom half

| First round | First round | First round | First round | First round | Second round | Second round | Second round | Second round | Second round | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Semifinals | Semifinals | Semifinals |
| 7 | M Polmans | 6 | 6 | | | | | | | | | | | | | | |
| | C İlkel | 1 | 0 | | 7 | M Polmans | 4 | 4 | | | | | | | | | |
| | J Jung | 4 | 5 | | | Y Shimizu | 6 | 6 | | | | | | | | | |
| | Y Shimizu | 6 | 7 | | | | | | | | Y Shimizu | 7$^9$ | 6$^1$ | 7$^7$ | | | |
| Alt | A Çelikbilek | 6 | 3 | 2 | | | | | | | C Wong | 6$^7$ | 7$^7$ | 6$^4$ | | | |
| | C Wong | 2 | 6 | 6 | | C Wong | w/o | | | | | | | | | | |
| | Z Kolář | 2 | 7$^7$ | 2 | 4 | J Duckworth | | | | | | | | | | | |
| 4 | J Duckworth | 6 | 6$^4$ | 6 | | | | | | | | | | | | Y Shimizu | w/o |
| Q | N Nakagawa | 6$^5$ | 6 | 2 | | | | | | | | | | | Q | Y Uchiyama | |
| | A Martin | 7$^7$ | 1 | 6 | | A Martin | 6 | 4 | 5 | | | | | | | | |
| WC | R Sakamoto | 4 | 3 | | | L Tu | 2 | 6 | 7 | | | | | | | | |
| | L Tu | 6 | 6 | | | | | | | | L Tu | 6$^1$ | 5 | | | | |
| Q | Y Uchiyama | 6 | 6 | | | | | | | Q | Y Uchiyama | 7$^7$ | 7 | | | | |
| WC | M Imamura | 1 | 1 | | Q | Y Uchiyama | 2 | 6 | 6 | | | | | | | | |
| | S-c Hong | 7$^8$ | 7$^8$ | | | S-c Hong | 6 | 4 | 4 | | | | | | | | |
| 2 | J Rodionov | 6$^6$ | 6$^6$ | | | | | | | | | | | | | | |