The Indians had a 100 percent strike rate with all three pugilists registering a win in their respective weight categories.
After Manoj, it was National Games bronze medallist Mandeep Jangra (69 kg) and reigning national champion Sukhdeep Singh (75 kg), who powered through to the round of eight.
Kumar, playing his second bout, made short work of his Sri Lankan opponent Mohamed Dilshan. Manoj was in his usual aggressive form from the start; his penetrating left and right jabs catching Dilshan off guard many times.
He was clearly the superior boxer and made it easy for the judges to declare him winner by a unanimous decision.
The 26-year-old Asian Championship bronze medallist is now one step away from another bronze and will have to out-punch Armi Khir of Malaysia next, in order replicate his 2007 success.