Shiva, up against second seed and local favourite Elnur Abduraimov, lost in a split verdict after a head-butt from his opponent resulted in a cut above his right eye.
It was in the final few seconds of the opening round when Shiva sustained the cut, forcing a stoppage by the referee.
Sumit, on the other hand, lost to Kazakhstan's top-seeded Vassiliy Levit in a unanimous verdict. Levit is the silver- medallist from last year's Rio Olympic Games.
"I am happy to have achieved the twin targets of a medal as well as qualification for the World Championships. This is my first international medal in lightweight and I always had the self-belief that I could achieve it," Shiva told PTI.
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The Assam-pugilist, a World Championships bronze- medallist in bantamweight (56kg), shifted to lightweight in December last year. He went on to win a gold medal at the National Championships before a couple of early losses in tournaments in Bulgaria and Thailand.
The 23-year-old defeated Olympic bronze-medallist, Asian Games gold-medallist and top seed Dorjnyambuu Otgondalai of Mongolia in the semifinal yesterday, during which he also sustained a cut above his eye.
"Obviously when you are competing against an established name, a few things play on your mind but then once you enter the ring, you have to block it out," he said.
There was also some disappointment for India with Gaurav Bidhuri (56kg) and Manish Panwar (81kg) losing their box-off bouts for World Championship slots.
While Gaurav lost to Japan's Ryomei Tanaka, Manish went down to Pakistan's Awais Ali Khan. Both the boxers were defeated in a split verdict.
The World Championships are due to be held in August- September and the top six boxers from each weight category will qualify for the mega-event in Germany.