Ranges captain Jose Ramires found the game's only goal, his 53rd-minute strike the difference between the sides in what proved a dour game.
The Suns had entered the game full of confidence after a 5-3 win against Melbourne City, but instead lacked their usual firepower as they were held goalless for the fifth time this season.
Coach Craig Carley said his side had lacked a spark in front of goal and had not taken the chances that had come its way.
"I thought it was a lacklustre performance if I’m honest," Carley said.
"I thought we did okay, but really lacked quality in the final third, too many players were off their games and were very reactive to situations.
"We certainly didn’t capitalise on set pieces and create clear-cut chances."
Carley was disappointed his side had still not learned to grind out results when having off days, something that has cost it a chance to compete at the top of the league this season.
"The first half I thought we controlled without being overly good, but the second half we dropped and we were disappointed to concede the goal in the manner we did," he said.
"We need to be better than that and should be beating teams like Whittlesea or at least grinding out a result if it’s not our day in front of goal."
The sixth-ranked Suns retain an eight-point gap to seventh-placed Murray United and will be guaranteed safety from relegation if they accumulate seven points in the season's closing four games, allowing for the sides ranked below them to win all their remaining games.
They will face Geelong this weekend away from home, confident of taking points against the NPL2 West's eighth-ranked side.
The curtain-raiser had the Suns' under-20s win for the first time in five games, winning a goal-fest 5-3.
Geordie Lelliott opened the scoring nine minutes in before Whittlesea's Daniel Cassar equalised on 25 minutes.
Sun Elijah Brooker made it 2-1 before half-time, but again Cassar had the answer, scoring early in the second half for 2-2.
But the Suns would take control with goals to Willie Sauiluma and Hammad Agog, while under-20s debutant Teng Kuol scored late after Whittlesea's Jake Stafrace had dragged a goal back.
After the breakthrough win, the Suns are seventh on the NPL2 East under-20 table.
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