You do have to be a monopoly in order for leveraging market share against competitors to be illegal.
You have a contradiction in terms - in a monopoly, the commodity is controlled by one entity. There are no competitors, there is no market share except 100%.
monopoly
Main Entry: monopoly
Pronunciation: \m-nä-p(-)l\
Function: noun
Inflected Form(s): plural monopolies
Etymology: Latin monopolium, from Greek monoplion, from mon- + plein to sell
Date: 1534
1 : exclusive ownership through legal privilege, command of supply, or concerted action
2 : exclusive possession or control
3 : a commodity controlled by one party
4 : one that has a monopoly
Thus, in a monopoly, there are no competitors. Antitrust lawsuits specifically deal with companies that do not have a monopoly. For example, see the cases vs Microsoft, where although MS did not have a monopoly on browsers, they did leverage their market share to unfairly crowd out competition and were sued for it.
We gave you an atomic bomb, what do you want, mermaids? -- I. I. Rabi to the Atomic Energy Commission