LONDON, July 15 (Reuters) - Venus Williams ran out of steam in her Wimbledon final defeat by Spain's Garbine Muguruza on Saturday, losing the second set 6-0, but she graciously refused to blame her loss on tiredness. After producing, as she has done throughout the tournament, clean-hitting and focused tennis, Williams failed to convert two set points at 5-4 and dramatically lost the next nine games as her hopes of a sixth Wimbledon singles title evaporated. The manner of her defeat inevitably led to speculation that the 37-year-old may have had a physical problem that stopped her from competing with the effectiveness she had shown in earlier rounds. In 2011, Williams was diagnosed with Sjogren's syndrome, an autoimmune disease that causes fatigue and joint pain but asked, at the post-match news conference whether that was a factor in her loss, she deflected the question, clearly not wishing to take away credit from her Spanish opponent. "She played really well.