It's a common misconception that you have to have a monopoly to run afoul of antitrust law. You don't. It also goes the other way--having a monopoly is not necessarily a violation of antitrust.

For instance, mergers have been shot down on antitrust grounds even when the merged companies would have had under 50% of the market.

Antitrust is more about the effects of actions on competition, rather than market share, although there tends to be a correlation between the two.