To celebrate his birthday, look back at #timbradley knockout victory of Joel Casamayor Next up for Top Rank Boxing, Two heated rivalries will come to a head Saturday, Sept. 10 in a historic championship doubleheader at the O2 Arena in London. The previously announced main event will see two-time Olympic gold medalist Claressa Shields, the WBC/WBA/IBF/Ring Magazine middleweight champion, aim to once again become undisputed at 160 pounds by facing longtime rival and WBO titleholder Savannah Marshall. Shields and Marshall faced off as amateurs, with Marshall edging Shields by decision at the 2012 AIBA Women’s World Championships in Qinhuangdao, China. Shields went 77-1 as an amateur and is the only female boxer to win undisputed titles in two weight classes. In the co-feature, Mikaela Mayer will look to get one step closer to becoming the first undisputed champ in her division by putting her WBO/IBF/Ring Magazine junior lightweight titles on the line against WBC champion Alycia Bau