Singapore won the first leg of their second round qualifier 5-3 against the Malaysians last Saturday to take a precious two goal advantage to Kuala Lumpur's Bukit Jalil stadium.
On Tuesday, it was reported that only 358 tickets out of the 2,000 allocated to Singapore fans were sold, meaning the Lions will have to tackle a Malaysian team backed by a strong partisan crowd in the second leg.
"It won't be easy at the Bukit Jalil," Hariss said in an interview with sponsors Nike.
"The Malaysians will makes all kinds of noise to get behind their team. The most important thing for us is to psychologically keep our heads in the game and focus on the job in hand, because we have an advantage on our side.
"We must not let it slip and we have to tighten things up and hit them when we can."
With Singapore likely to defend their two goal cushion in numbers, the Lions will look to players like Hariss to put in the work to support the attack whenever possible.
"I can do a lot more in the attacking third," he said.
"Supporting the attack is one part of my game I'm looking to improve."
He added: "On an attacking note, we were all over them [in the first game]. We really made use of our attacking players, the good holding players and good ball-playing players. The experience of Shahril [Ishak], Qiu Li, Jiayi and [Aleksandar] Duric showed.
"We showed that we could get the goals but we need to get things tighter all over the pitch, especially in defence and midfield, That's the most important thing for the second game."
Malaysia captain Safiq Rahim will miss the game after being sent-off for a nasty challenge on Hariss.