Takeuchi was the only overseas player in the last eight stage. Sixth-seeded Gajjar now faces Ashutosh Singh who upset national champion and seventh-seeded Ranjith Virali-Murugesan 4-6, 6-4, 7-6(3).
Tournament favourite Yuki Bhambri, too, eased into the last four with a 6-2, 6-4 victory over Divij Sharan. The junior Australian Open champion will now take on second seed Vishnu Vardhan, who got the better of eighth-seeded Rupesh Roy 6-2, 7-6(7).
But the day belonged to Gajjar, who fought back from a set down to take the match against the 21-year-old Japanese in two hours and 20 minutes. The two baseliners tested each other with solid groundstrokes, though it was Gajjar who clearly had the edge.
Takeuchi served out the first set with a break in the fifth game but Gajjar bounced back in the second set. Neither player dropped serve to take the set to the tie-breaker, where Gajjar prevailed to make it a set all.
A confident Gajjar began the third set by breaking Takeuchi in the first game and kept up the pressure till the last. He won the set and the match, breaking Japanese in the ninth game.
Yuki, seeded fourth, was sublime in his defeat of Sharan, with whom he had been hitting quite a bit. The 16-year-old from Delhi began with a break in the very first game and followed it up with another break in the third to win the first set.
Sharan cut down on his errors in the second set but Yuki managed to steal a decisive break in the fifth game to close the match in nearly an hour and a half.
