After finishing as third seed in the round-robin stage, Alok met compatriot and sixth seed B Bhaskar in the quarterfinals.
Bhaskar cracked with a break of 70 points to win the first game. Alok immediately countered and set the match on level 1-all by winning the next game.
In the third game, Alok missed a dicey cannon on 79 points, and Bhaskar scored the remaining 58 points to take a 2-1 lead.
After that Alok produced three consecutive breaks of 96, 74 and 88 points in next three games to knock out Bhaskar and confirm his berth in medal round.
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The other semifinal will be played between reigning world champion Rupesh Shah of India and Thawat Sujaritthurakarn of Thailand.
Despite leading 3-2 Rupesh was under the threat of losing the match against Asian Games silver medallist Nay Thway Oo.
After winning the sixth game, Nay Thway Oo elevated the score to 87-20 in decider. There was nothing much left in the match but Nay Thway unexpectedly played a loose centre pocket in-off and missed to give one last chance to Rupesh.
In the 14th Asian Under-21 Snooker Championship, the round-robin stage ended today with 16 players from four groups qualifying for the pre-quarterfinals. These included three Indian players.