Changes to the regulations for mandating the use of turbo-hybrid engines came at the start of a highly successful period for Hamilton, during which he has won six further drivers' titles. Hamilton won consecutive titles in and during an intense rivalry with teammate Nico Rosberg to match his hero Ayrton Senna's three World Championships. Following Rosberg's retirement, Ferrari's Sebastian Vettel became Hamilton's closest rival in two intense championship battles, and Hamilton twice overturned mid-season points deficits to claim consecutive titles again in and . Hamilton won his third and fourth titles in a row in and , respectively, equalling Schumacher's record of seven drivers' titles.