First of all, only opened is a past participle. It requires the -ed ending.
The verb open is a dynamic verb. It refers to a change in someone’s or something’s state or condition from closed to open. The adjective open, on the other hand, refers to a state or condition.
I find your first sentence a bit odd. I need some context that tells me that there was a change in the state of the competition. A past tense verb would at least do that.
The second sentence is more natural. It is describing a state.
Here are a few further examples.
All of these refer to a change in the state:
Someone opened the door.
The door was opened by someone.
The door was opened.
Khalil opened the window because the room was hot.
New York’s newest subway line was opened in May 2015. (It began service in May 2015.)
My great-grandfather opened an inn in 1860. (He started the business in 1860.)
My father opened a flower shop in 1944.
These sentences refer to a state:
When we came to the room, we found that the door was already open.
Khalil left the window open when he left the room.
New York’s newest subway line has been open for seven months.
My great-grandfather’s inn was open for 30 years. (It went out of business in 1890.)
The flower shop has been open for 71 years. It is still open; my brothers run it now.