While Manoj thrashed Tajikistan's Rakhimov Shavkatdzhon, second seed Vikas got past Korea's Lee Dongyun in the quarterfinals by a similar margin of 3-0.
However, Sumit Sangwan (81kg) lost to top seed Petr Khamukov but continues to be in the hunt for a quota berth as the fifth available spot in his category will go to the lucky quarterfinal loser, whose opponent goes on to win the gold medal.
"Given the terrible state in which boxing is right now, I am relieved to have qualified for the Games. There was a lot of pressure on the team which is competing here and I am glad that I came good," Manoj told PTI.
"I am immensely thankful to the coaches, the Sports Ministry, the national coaching staff, the Sports Authority of India and my personal coach Rajesh, who is also my elder brother. Their efforts have helped me pull it off," he added.
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"I am extremely delighted to have secured the Olympic quota. It has been a long journey with quite a few ups and downs. I was confident in my abilities and always believed that I had what it took to reach the Olympics and put up a strong performance on that stage. My coaching staff along with my backers JSW have always had faith in me and I am glad that I have managed to justify that when it mattered," he said.
Vikas and Manoj took the total number of Indian boxers assured of Olympic berths to three as Shiva Thapa (56kg) had made the cut for Rio during the Asian Qualifiers in March.
National coach Gurbax Singh Sandhu described the duo's performance today as dominating.
"Manoj dominated all three rounds and boxed intelligently to win and qualify for the Games," Sandhu said.
Vikas, however, is a doubtful starter at tomorrow's semifinal as he has sustained a cut on his face. Both Vikas and Manoj had competed in the 2012 London Games as well. While Manoj had lost in the quarterfinals, Vikas was ousted in the preliminaries.