sentences:  Previous heavyweight eras had multiple boxers considered world class fighters competing, such as Joe Frazier, Muhammad Ali and George Foreman during the 1970s, and Mike Tyson, Lennox Lewis and Evander Holyfield during the 1990s.According to some observers, this was not the case for the Klitschkos, who were able to dominate other heavyweights with relative ease.The list of fighters the Klitschkos have defeated, however, includes unified world cruiserweight champion David Haye, long-time top ranked contender Alexander Povetkin and two-weight world champion Tomasz Adamek.