But Nishino's squad, which was the lowest-ranked and considered the longest shot to advance from the group, was emboldened by its opening 2-1 win over Colombia and now is aiming to advance with a bigger win over winless Poland, who are already out of contention.
"It was an ultimate decision for me to make".
He said his decision to make six changes to his side was down to fatigue rather than any attempt to play for a draw.
Nishino made six changes to the starting lineup ahead of the match, saying some of his players were tired.
Poland are out of the tournament after two defeats, and they will have nothing more to play for other than pride.
Furious fans hit out at the Japanese approach on social media, with some even going as far to suggest they should be thrown out of the tournament for apparent collusion.
Japan and Senegal finished with the same number of points, the same goal difference and the same number of goals scored.
With two fewer yellow cards, Japan's tally of minus-four points was two better than Senegal, who may feel aggrieved at being sent home for a reason other than the quality of their football.
"The manner in which the team qualified was not the initial goal", said Nishino afterward.
Japan faces Group G victor Belgium on Monday in Rostov-on-Don. His cleverly directed header looked destined for the bottom corner until Kawashima made up huge amounts of ground to claw it to safety with a right hand at full stretch.
Senegal, however, has become the first victim of the new tiebreaker.
The result means Japan reach the knockout phase for the third time in their six World Cups and with Nishino undoubtedly about to face some hard questions about his decisions for the match at a sweltering Volgograd Arena. The Senegalese side couldn't land the killer blow, and as full time blew in that game, Japan's spot in the Round of 16 was confirmed.
Japan has failed to score in 4 of the 5 previous World Cup matches against European teams with their last goal coming in a 3-1 victory over Denmark.