Marko Ostoja has found clay courts difficult, recording 55–74 across 129 matches (42.6%). The numbers point to a surface that has not consistently suited the game — a key area of opportunity on the calendar. Claimed 1 title on clay: Brussels-2.

At Grand Slam level (Roland Garros): Marko Ostoja is 5–5 (50.0%) across 10 Grand Slam matches — below .500, though the elite draw depth at the Slams makes that a notoriously difficult barrier to crack.

One final reached on clay, converted into a title — a perfect finals record on this surface. 2 semifinals. 4 quarterfinals.

vs. Top 10 on clay: 1–8 (11.1%, 9 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: 11–8 (57.9%); best-of-three: 44–66 (40.0%). Significantly better in five-set matches — a strong physical profile on clay that tends to tell as matches and tournaments progress.

Best season: 1983 — 9–5 (64.3%) from 14 matches. The best single-season display to date — a useful reference point as the clay record continues to develop.