Sinha, ranked more than 160 places below his Japanese rival, dropped the opening set before gathering his wits to carve out a shock 4-6 6-2 7-6(7) victory in the quarter finals.
Joining Sinha, a lowly 294 in the world junior rankings, in the last four were his compatriots - top seed Siddanth Banthia and Megh Bhargav Patel - and Malaysia's Christian Didier Chin who ousted India's seventh-seeded Dhruv Sunish.
Mahak Jain breezed past compatriot and wild card Salsa Aher 6-2 6-2 while Mihika also scored a easy win over Funa Kozaki of Japan 6-1 6-2 in today's quarter final ties.
In the other quarter-finals, Taipei's Kuan Yi Lee stopped the run of Indian Vaidehi Chaudhari 6-4 6-4 while Thailand'a Mai Napatt Nirundorn got past Mana Kawamura of Japan 6-1 1-6 7-5 to complete the semi-final line-up.
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The results:
Boys singles (quarter finals): 1-Siddhant Banthia (Ind) bt Ho Ray(TPE) 3-6 6-3 6-1; Megh Bhargav Patel (Ind) bt Digvijay Singh (Ind) 6-4 6-4; Christian Didier Chin (MAS) bt 7-Dhruv Sunish (Ind) 7-6(2) 6-4; Nitin Kumar Sinha (Ind) bt 3-Taisei Ichikawa (JPN) 4-6 6-2 7-6(7).
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