Sourabh beat Igarashi 25-23, 24-22 on Thursday to set up a quarterfinal clash with home favourite Tien Minh Nguyen.
Verma, the reigning national champion, got a bye in the first round and beat Japan's Kodai Naraoka 22-20, 22-20 in the second. He is now the lone Indian left in the tournament following the ouster of Siril Verma and Shubhankar Dey earlier on Thursday.
China's Lei Lan Xi beat Siril 21-12, 21-15 to halt his march to the quarters. The 97th ranked Indian had earlier stunnded World No. 22 Darren Liew 17-21, 21-19, 21-12 in the second round.
Shubhankar Dey, on the other hand, lost 11-21, 17-21 to unseeded Malaysian Jia Wei Tan.
In men's doubles, Arun George and Sanyam Shukla lost 13-21, 11-21 to top seeds Lu Ching Yao and Yang Po Han of Chinese Taipei.