back after being down a round to outwit their respective rivals. While Asian champion and fourth seed Shiva got the better off Argentina's Alberto Melian 2-1, sixth seed Manoj beat Canada's Yves Ulysse by the same margin.
However, former cadet world champion Thokchom Nanao Singh (49kg) and Asian Games silver-medallist Manpreet Singh bowed out of the event after losing their prequarterfinal bouts to more fancied opponents.
Nanao lost 0-3 to eighth seed Anthony Chacon Rivera of Puerto Rico, while Manpreet went down to top seed Teymur Mammadov by the same margin.
Shiva will face Azerbaijan's Javid Chalabiyev in the quarterfinals, while Manoj will be up against Cuba Yasnier Lopez, who stunned third-seeded Russian Armen Zakaryan. A win in the quarters, scheduled for Wednesday, will assure both of at least a bronze medal in the mega-event.
"It was a very tough bout and took a lot out of me but I somehow pulled through," Shiva told PTI as he caught his breath after the draining contest.
Shiva, a silver-medallist at the 2010 Youth Olympics, faced an opponent whose tactics were not too different from his own.
"I tried to play it cool in the opening round and it could have gone either ways because we were quite similar in our style. It was a 50-50 thing. He clinched it and I increased my tempo in the next two rounds," Shiva said.
National coach Gurbax Singh Sandhu, while lauding Shiva, said it was one of the best bouts he has witnessed.
"It was a very close bout and an excellent one at that. Both wanted to counter-punch, both of them had good footwork. But Shiva dominated the third round, which he won unanimously," said the long-serving coach.
However, it was the end of campaign for Nanao, who also endured a draining bout.
"Nanao also boxed very well but I guess the other guy managed to get ahead because of some uppercuts that he connected very clearly. I think Nanao was very good with his overall technique but then it happens," Sandhu said.
The Manipuri-lad, a former cadet world champion besides being an Asian Championships silver-medallist, lost 0-3 on points. Rivera was warned once in the third round but that could not help Nanao's cause.
In evening session, Manoj opened the proceedings for India. The two-time Asian Championships bronze-medallist was down by three points after the opening round.
However, he roared back into the contest to clinch the second round on a split decision before dominating the final three minutes to complete the turnaround.
"Manoj picked up in the second and third round after being a little slow off the blocks. He was very aggressive and fought in tempo style and completely dominated in the third round," said Sandhu.
But Manpreet could not repeat this magic against world number one Mammadov, who clinched each of the three rounds to be the unanimous winner.
"Manoj got a cut on his forehead in the second round which was pretty deep but he did not give up. The doctor stopped the bout in the third round. But this was the best performance I have seen of Manpreet even though he did not win," Sandhu explained.