That's not right either, as you have made the (false) assumption that each of Nadal, Djokovic, Haas and Wawrinka have 25% chance to reach the final from the top half...
Assume Nadal has 4 times the chance of reaching the final as Haas, and Djokovic has 3 times the chance of reaching the final as Haas, and Wawrinka has equal chance of reaching the final as Haas.
So it would be [(0.5 + 0.5 + 0.5 + 0.5 + 20 + 20 + 20 + 60 + 70) / 4 ] * 0.5
This is equal to 24%
Which is almost 25%, or one-quarter, and since he is in the semis that makes sense