The 20-year-old Mochizuki, ranked 215th in the world, trailed 5-2 in the third set before forcing the deciding tiebreaker. It was his first win over a top-10 player.
“I don’t really know what is happening right now,” said Mochizuki, who advanced to the quarterfinals for the first time. “I lost the first set so easy, so quick. Everyone knows he is a great player but I kept fighting and that is all I could do. I am so happy. I don’t know how, but I won the match.”