Level playing field
Fair competition, where no advantage is shown to either side.
This phrase clearly alludes to the requirement for fairness in games which are played from end to end and where a slope would give one team and advantage, e.g. football. The figurative use of the phrase isn't especially old and the first record I can find of it is from the Tyrone Daily Herald, January 1977:
"Our philosophy is that we have no problem competing with the mutual savings banks if they start from the level playing field," Bolger said. [John Bolger, lobbyist for the US Bankers Association]
This harks back to another American phrase, from about a century before - on the level. This is first recorded in George Burnham's Memoirs of the United States Secret Service, 1872:
"On the level, meeting a man with honorable intentions."
See other phrases that were coined in the USA.