However, the #25 Holden Commodore pilot has conceded he can't afford to lose more ground if he wishes to contend for the Repco Supercars Championship crown.
Mostert and van Gisbergen each walked away from the Melbourne 400 with two wins apiece, though it was the Kiwi who scooped the Larry Perkins Trophy.
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Mostert wound up just 17 points shy of van Gisbergen by the weekend's close.
Although van Gisbergen reigned supreme, extending his championship advantage to 144 points over Anton De Pasquale, Mostert said he doesn't feel he's far away.
“The weekend was tough,” Mostert said on Supercars Trackside.
“We had to chase it [the car] a lot… practice, qualifying, all that, our car where we ended up the weekend is a lot different to where we would normally roll out at one of our generic tracks that we'd go to.
“We've still got work to do. You see how quick Shane is out there, I thought we were a bit better on the hard [tyre], but most of our races are on the soft.
“I think his car just really suited here for whatever reason. I don't think he has that much buffer on us at other tracks we go to, except for say Tasmania – he was absolutely crazy there.
“We've got work to do, 100 percent. How quickly we can keep trying to make that gap [smaller], it's crucial. It's crunch time now.
“When Shane is on, he's in the top three every race. Even if he starts 23rd or whatever it is. To try and steal that championship away, we can't give away any more.
“We've got to really turn up at the next round and keep pushing to the end of the year. He's just got momentum.”
Mostert did get the edge over van Gisbergen in the weekend's closer, Race 9.
That came after the 30-year-old executed an overcut strategy to emerge in front of the New Zealander in the pit stop sequence.
Pushing to get past Mostert, van Gisbergen suffered a left-front lock-up. Such was the resulting flat spot that it began to expand until the outer tyre tread let go.
Mostert credited his Walkinshaw Andretti United crew for the strategy and pit stop execution that put him in a position to heap pressure on van Gisbergen.
“At the start of the race, I didn't feel that comfortable on the soft,” he explained.
“Our balance, just didn't get the maximum out of the car and Shane was coming like a bullet. I really wish I had more to fight with for him when he passed me.
“Obviously we got to the pits and as he passed me he went into the pits so we went one lap longer.
“The guys on the guns in the lane are unbelievable. This race win is from those guys, they put everyone else on notice out there.
“When we came out to Turn 1 there was a bit of breathing room. I was able to bring the tyre in. Shane was pushing pretty hard, but you've got to get the track position.
“You've got to go full pace and made him make that little mistake.”
Mostert holds third in the championship, 162 points in arrears of van Gisbergen after nine races.
Round 4 of the season takes place across April 30-May 1 at Wanneroo Raceway for the Bunnings Trade Perth SuperNight.