Defending champion Taylor Fritz tumbled out of the Japan Open on Thursday after a stunning second-round loss to Japanese wild card and world number 215 Shintaro Mochizuki.

Top-seed Fritz, the highest-ranked American in the world at number 10, lost 0-6, 6-4, 7-6 (7/2) to an opponent without a tour-level win to his name before this tournament.

Fritz, who has won two titles this season, looked to be cruising into the quarterfinals when he took the first set in just 29 minutes.

But the momentum shifted when Mochizuki broke his opponent midway through the second set, and the 20-year-old fought back from a losing position in the third to force a tiebreak.

Mochizuki closed the match out with a volleyed smash at the net, setting up a quarterfinal match-up with Australian Alexei Popyrin, who beat Chile's Cristian Garin 4-6, 7-6 (7/3), 6-2.

Fritz's exit meant the top three seeds in Tokyo have all been eliminated, with Norwegian No 2 Casper Ruud and German No 3 Alexander Zverev also bowing out early.

No 4 seed Alex de Minaur progressed to the quarterfinals with a 6-0, 7-5 win over Argentina's Diego Schwartzman.

The Australian said he was not surprised that so many of the tournament's big names had fallen by the wayside.

"We've all played a lot of matches this year and it's been a long year," said de Minaur.

"At times I feel like these are the types of tournaments that are more prone to surprises.

"I'm just happy I was able to keep my head down and play at a good level."