BRUSSELS—A sculpture of a raised middle finger greets visitors to the elegant offices of Margrethe Vestager, the most powerful woman in Brussels. It is a reminder, from a disgruntled lobby group in her native Denmark, that tough decisions come with the territory.

One hundred days into her new job, Europe’s antitrust chief is preparing a series of politically explosive decisions involving some of the world’s biggest companies, including Google Inc., Apple Inc. and Amazon.com Inc.

Perhaps her most incendiary case is a 2½-year-old probe into Russian energy company Gazprom OAO . In an interview, Ms. Vestager said she will move against the state-owned company shortly despite the Ukraine crisis, opening up the possibility of multibillion-dollar fines.

“I think we can move the case forward in a hopefully relatively short time span,” she said. European Union authorities have been investigating whether Gazprom abuses its market dominance in Eastern Europe to block rivals, but settlement talks have been on ice since violence flared in Ukraine a year ago.

“It’s very important for me to make sure that any company in the European market is being faced with the same set of rules and the same effort of enforcement,” Ms. Vestager said.