Sergi Bruguera defeated Mark Koevermans 6-3 6-3 in the Estoril round of 32 on clay. The upset overturned the form book — Mark Koevermans came in leading the head-to-head 1–0, defending last year's round of 32. Sergi Bruguera claimed a first win over Mark Koevermans in two meetings, narrowing the head-to-head to 1–1.
| Date | Tournament | Round | Surface | Winner | Score |
|---|---|---|---|---|---|
| 1991-05-06 | Hamburg | R16 | clay | Mark Koevermans | 6-4 6-3 |
Averages over each player's completed matches in the selected window. ATP, WTA & Challenger.