Jun Kuki has found clay courts difficult, recording 56–91 across 147 matches (38.1%). The numbers point to a surface that has not consistently suited the game — a key area of opportunity on the calendar. Jun Kuki has reached 2 finals without yet claiming a title on clay — one of the finest margins in tennis.

At Grand Slam level (Roland Garros): Jun Kuki has struggled at Grand Slam level on clay: 4–9 (30.8%) in 13 matches. The best-of-five format and elite fields make this the toughest benchmark on this surface.

2 finals reached on clay — none converted into a title yet. Reaching 2 finals is a mark of real quality, but the gap between finalist and champion is one of the finest lines in the sport. 3 semifinals. 6 quarterfinals.

vs. Top 10 on clay: 0–10 (0.0%, 10 matches). Top 10 opponents have represented a clear ceiling on clay; addressing that deficit is the single biggest lever for improving the overall record here.

By format on clay — best-of-five: 3–14 (17.6%); best-of-three: 53–77 (40.8%). Markedly stronger in three-set formats; the win rate drops noticeably in five-setters, which has direct implications for Grand Slam performance on clay.

Peak season: 1976 — 19–17 (52.8%) from 36 matches. That year captures the ceiling of what Jun Kuki can do on clay when performing at their best and represents the standard to aim for.