With Spanish construction group ACS turning the biggest M&A deal announced so far this year into a potential takeover battle, a little-known advisory firm has stepped into the spotlight.

The Spanish group and its German subsidiary, Hochtief, is being advised on a bid for national toll road operator Abertis Infraestructuras by JPMorgan and Lazard, neither of which is a stranger to big deals. But it is also being helped by Key Capital Partners, a seven-year-old corporate finance firm based in Madrid that has built a team around top talent from many much bigger banks.