The first round was very tight, although Ouadahi's better quality was reflected by the judges' 4-3 scoring in his favour.
The Algerian got the better of the early part of the second stanza, but his constant movement was proving to be a drain on his energy.
As the contest wore on, Shimizu started to take the upper hand and was still only trailing by one point as they entered the final three minutes.
A public warning for Ouadahi in the third round gave Shimizu a two-point boost to his tally. However, he started to tighten up and became very cautious.
That allowed Ouadahi to catch his breath and he landed a few scoring blows to ensure that the result would be close.
However, it was Shimizu who scraped a 17-15 win to guarantee himself a spot on the rostrum.