Table:
text: Àlex Corretja
Wins (17–13)
Wins (17–13)
Outcome Date_ Championship Surface Opponent_in_the_final Score_in_the_final
Runner-up 28 April 1997 Monte Carlo, Monaco Clay Marcelo Ríos 4–6, 4–6, 3–6
Runner-up 5 May 1997 Munich, Germany Clay Mark Philippoussis 6–7, 6–1, 4–6
Winner 19 May 1997 Rome, Italy Clay Marcelo Ríos 7–5, 7–5, 6–3
Winner 21 July 1997 Stuttgart Outdoor, Germany Clay Karol Kučera 6–2, 7–5
Winner 16 February 1998 Dubai, UAE Hard Félix Mantilla Botella 7–6 (7–0) , 6–0
Runner-up 11 May 1998 Hamburg, Germany Clay Albert Costa 2–6, 0–6, 0–1, ret.
Runner-up 8 June 1998 French Open , Paris, France Clay Carlos Moyà 3–6, 5–7, 3–6
Winner 13 July 1998 Gstaad, Switzerland Clay Boris Becker 7–6 (7–5) , 7–5, 6–3

Question: What type of surface did they play on for Àlex Corretja: Wins (17–13) on July 21, 1997?