Axelsen seemed decidedly at a loss in the first set, while the energetic Tanaka appeared to be the complete opposite.
Axelsen, a double Olympic champion, fought his way back into the match, and in a close and dramatic third set, he won the decisive points in a close finish.
The Danish badminton star is on his way back to his usual level after largely not playing badminton for the past six months.
"It wasn't the prettiest badminton. I don't know how I won this match, because I felt like an elephant in the first set", said Viktor Axelsen to TV 2 Sport. "I'm damn proud that I won this match, if I'm being completely honest", he said.
Back in April, Axelsen underwent surgery for a back injury, and it was only a little over a month ago that he made a comeback at a tournament in Hong Kong.
Now he is the only Danish men's singles player left at the Denmark Open after both Anders Antonsen and Rasmus Gemke lost their matches in the second round earlier on Thursday.
The World No. 2 Antosen simply couldn't get going against Sen, who perhaps played the game of his life and made few unforced errors against the favored Dane.
"Lakshya played a good game, and he came up with some great shots. Hats off to Lakshya for playing really well. He was simply better", said Antonsen after the defeat to TV 2 Sport.
"It's really devastating, but I also have to recognize how he played", said Antonsen.
