Hasegawa headed home from a Michael Glassock cross in the 93rd minute to give Olympic their second win on the trot and extend their unbeaten run to four.
It was his second of the season, and a goal that could prove huge come end of the season when round 22 concludes.
It was a heartbreaking result for the Young Sky Blues, who had their chances to take the lead throughout the second half but paid the price for their poor finishing.
The result boosts Olympic’s finals hopes with another vital three competition points with only six games left to play in the regular season.
After the game, Sydney Olympic coach Ante Juric acknowledged that a draw perhaps would have been a fair result but was more than happy to celebrate the win.
“It’s nice to get two wins in a row now because it’s the business end of the season,” he said.
“In terms of football, it probably should have been a draw because they were very good, but that’s Nick’s [Sorras] job and that’s why he got brought back into the team.
“Sydney FC had some good chances, but they did miss them, and we were a bit lucky to get one at the end. The win is important because it’s the business end of the year, and we have a good spirit and are building slowly.”
