laughter] >>> they might be two of the most famous on screen boxers, sylvester stallone as underdog ricky balboae screen for "grudge match" they put on the kid gloves with me for their "nightline" interview. >> your punch. >> there is not much that can match the thrill learning how to throw a punch from the most iconic movie boxer of all time. but it gets even better, buzz to my right the other most famous big screen fighter of our generation. >> push with your shoulder. >> um. >> whoa. >> reporter: in one corner, sim vester sta len as rocky. in the other corner, robert de niro as jake lamata as raging bull. now all the years later they're in the ring together in the movie "grudge match." the concept two old rivals decide they need to settle a score >> please i am becki inbegging . begging you. >> reporter: throwing punches at each other and taking a few jabs at the roles that made them famous. whutz id whose idea was it? >> wasn't my idea. >> it is fooling with two boxing icons of our generation, the generation before, and probably the next generation. >> it's raging bull and rocky. and head-on