TOKYO, July 31 (Reuters) - The United States" hopes of a first gold medal in more than half a century in the 800 metres got a boost on Saturday when 19-year old Athing Mu won her semi-final race with a time of one minute and 58.07 seconds, to propel her into medal contention early next week. The absence of South Africa"s double champion Caster Semenya, ruled ineligible due to heightened testosterone levels, has opened the field to new contenders. read more Leading that charge is Mu, the fastest in the world this year with 1:56.07, who will be hunting for her country"s second gold in the event after Madeline Manning"s triumph in 1968 and their first medal since 1988. "It"s amazing. The goal was always to make the final and now I"m ready to go," she said after the semi-final. Other contenders for medals includes Britain"s Keely Hodgkinson, 19, who came from behind to impressively win her semi ahead of China"s Chunyu Wang, whose personal best of 1:59.14 was good enough to propel her to the final. On Tuesday, they will face Jamaica"s Natoya Goule, the 2019 Pan-American gold medallist, who got through to the final with another confident performance in the qualification rounds to progress with a time of 1:59.57, ahead of another British runner, Jemma Reekie whose 1:59.77 secured her spot in the medal race. Ethiopia"s Habitam Alemu, who qualified second in her race in 1:58.40, will be a potentially dangerous outsider for a podium finish next week.