I'm editing a lot of Olympics content right now, and here's a word that caused me to pick up my AP Stylebook to double check: semifinals.
I wasn't sure if that word needed a hyphen or not. But one check of my AP Style handbook confirmed my suspicion: no hyphen.
i.e.: "She qualified for the semifinals in two events: the 400-meter freestyle and the 50-meter freestyle."
The entry in the AP Stylebook for "semi-" says: The rules in prefixes apply, but in general, no hyphen.
* Also, here's the link to the 2012 Olympic guidelines from AP Stylebook.
Here are some other "semi" words to brush up on:
semiannual –– AP Stylebook says: twice a year, a synonym for biannual. Do not confuse it with biennial, which means every two years.
semitrailer –– AP Stylebook says: Or tractor-trailer, but not semi-tractor trailer.