After snatching the opening two games for a comfortable 2-0 lead in an 87-minute thriller, Lewis looked set to wrap it up after leading 8-4 in the third. But the 26-year-old from Leicester lost his way and allowed fourth seed Adnan to get back into the match, as the Malaysian forced the decider.
"I should have won the third," said the delighted Englishman after his win. "But it was a big match for me and I got nervous after that lead in the third game. I think I got too cautious and it allowed Adnan to come back," explained the fifth seed.
"It was the same in the fourth. But I was more attacking in a very close fifth game. He had two match balls and in the end I was just happy to survive."
Lewis will take on top seed Ritwik Bhattacharya after the Indian overcame qualifier and compatriot Siddharth Suchde 11-6, 8-11, 11-4, 7-11, 11-3.
The other semi-final will feature Malaysia's third seed Timothy Arnold and India's No2 seed Saurav Ghosal after both fashioned easy quarter-final wins over fellow countrymen.
RESULTS: Otters Open Squash Championship, Mumbai,
India
Quarter-finals:
[1] Ritwik Bhattacharya (IND) bt [Q]
Siddharth Suchde (IND) 11-6, 8-11, 11-4, 7-11, 11-3
[5] Darren Lewis
(ENG) bt [4] Mohd Nafiizwan Adnan (MAS) 11-5, 11-6, 9-11, 9-11, 11-10
(3-1) (87m)
[3] Timothy Arnold (MAS) bt [Q] Shahril Shahidan
(MAS) 11-6, 11-7, 11-9
[2] Saurav Ghosal (IND) bt Gaurav
Nandrajog (IND) 11-7, 11-3, 11-8