Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Date | Tournament | Surface | Opponent | Score |
| --- | --- | --- | --- | --- |
| September 3, 2001 | Aschaffenburg , Germany | Clay | Martin Verkerk | 7–6, 6–2 |
| January 27, 2003 | Dallas , United States | Hard (i) | Justin Gimelstob | 6–3, 7–6 |
| July 4, 2005 | Oberstaufen , Germany | Clay | Albert Portas | 7–5, 6–2 |
| May 8, 2006 | Dresden , Germany | Clay | Janko Tipsarević | 7–6, 6–2 |
| May 29, 2006 | Ettlingen , Germany | Clay | Michael Berrer | 6–4, 6–3 |
| July 3, 2006 | Córdoba , Spain | Hard | Kevin Kim | 6–7, 6–1, 7–6 |
| June 18, 2007 | Almaty , Kazakhstan | Clay | Daniel Brands | 6–4, 6–2 |
| June 25, 2007 | Almaty , Kazakhstan | Clay | Woong-Sun Jun | 6–3, 6–2 |
| August 31, 2008 | Freudenstadt , Germany | Clay | Matthias Bachinger | 6–3, 6–4 |
| September 7, 2008 | Alphen aan den Rijn , Netherlands | Clay | Iván Navarro | 6–4, 6–3 |
| September 20, 2009 | Todi , Italy | Clay | Adrian Ungur | 2–6, 6–1, 7–6 (8–6) |
| June 11, 2011 | Košice , Slovakia | Clay | Victor Crivoi | 6–2, 6–1 |
| October 28, 2012 | Porto Alegre , Brazil | Clay | Gastão Elias | 2–6, 7–6 (7–5) , 7–5 |
Question:
On what surface was the opponent Michael Berrer?
Answer:
Clay