Serena speeds to 50th win of season
After Zakopalova lost the first eight games, the third-seeded Czech managed to break Williams' serve twice in the second set to tie it at 4, but the top-ranked Williams broke back to take the set and the match.
''I was hardly making mistakes in the first set,'' Williams said. ''I have to figure out a way to continue that tennis for more than one set, and I'm definitely going to go back and watch the film.''
According to the WTA, the most wins Williams previously had going into the summer hard-court season was 38 in 2003.
In the other semifinal, Sweden's Johanna Larsson came from behind to overpower Italy's Flavia Pennetta 2-6, 6-3, 6-4.
Williams is going for her seventh title of the year and 53rd of her career, while Larsson is seeking her first.