While Pillai beat Yuet Wa Yuen of Hong Kong, Sanchis got the better of Hymen Choi of South Korea.
Another Indian in fray, Asutosh Padhy also made it to the quarterfinals, beating Ka Wai Cheung of Hong Kong 4-2.
Another local cueist Amee Kamani defeating compatriot Meenal Thakur 2-0 in just 45 minutes to progress to the next round.
Amee, however, now runs into Thailand's Sripaporn Nuanthakamjan, one of strong contenders for the crown.
Waratthanun made a beak of 73 in the second frame and Sripaporn made a break of 51 in the second frame to blank their opponents 2-0 in a best of three frames.
Fan Zhengyi of China fired breaks of 81, 82 and 74 in the first, third and fifth frames to win the match 4-1 with great authority and advance to the quarterfinals of the Asian Under-21 Snooker Championship.