Table:
text: Serena Williams
Singles: 21 (17 titles, 4 runner-ups)
Singles: 21 (17 titles, 4 runner-ups)
Outcome Year_ Championship Surface Opponent Score
Winner 1999 US Open Hard Martina Hingis 6–3, 7–6 (7–4)
Runner-up 2001 US Open Hard Venus Williams 2–6, 4–6
Winner 2002 French Open Clay Venus Williams 7–5, 6–3
Winner 2002 Wimbledon Grass Venus Williams 7–6 (7–4) , 6–3
Winner 2002 US Open (2) Hard Venus Williams 6–4, 6–3
Winner 2003 Australian Open Hard Venus Williams 7–6 (7–4) , 3–6, 6–4
Winner 2003 Wimbledon (2) Grass Venus Williams 4–6, 6–4, 6–2
Runner-up 2004 Wimbledon Grass Maria Sharapova 1–6, 4–6
Winner 2005 Australian Open (2) Hard Lindsay Davenport 2–6, 6–3, 6–0
Winner 2007 Australian Open (3) Hard Maria Sharapova 6–1, 6–2

Question: What's the surface when the championship is Australian Open Singles: 21 (17 titles, 4 runners-up) for Serena Williams?