The AVF racer finished 0.016 seconds ahead of Lotus driver Roy Nissany, thanks to posting a quickest lap of 1m59.745s at his final opportunity.
"I had one set of new tyres, like everybody, and the first two laps I had yellows. [Giuseppe] Cipriani was off at Pouhon and [Egor] Orudzhev at Brussels," Dillmann explained.
"I had one more lap. It was not a mega lap, but I needed to keep it together otherwise I was going to finish in the 2m02s or something.
"I think it will be a high-downforce set-up today because we are expecting it to be wet; there's a 90 per cent chance of rain at four o'clock."
An outside front row grid position represents Nissany's highest grid position in the category.
The Hungarian-raised Israeli had completed just one lap of Saturday's opening race before retiring with a broken driveshaft.
In a tightly contested session, Arden's third place qualifier Aurelien Panis lapped just 0.071s shy of Dillmann's benchmark.
The Frenchman's team-mate, and Spa race one winner, Orudzhev will line-up from the penultimate position on the grid after coming to a halt on track.
Star of last year's Formula Renault 3.5 Spa meeting, Matthieu Vaxiviere held provisional pole until the chequered flag, but the SMP Racing man was demoted to fourth following a spate of last gasp improvements.
RP Motorsport's Vitor Baptista outshone more experienced team-mate Johnny Cecotto Jr to finish fifth, ahead of Aragon winner Louis Deletraz.
At the conclusion of the used tyre runs at the halfway stage of the session, Alfonso Celis Jr's time of 2m01.301s was sufficient to top the timing screens ahead of Rene Binder.
Celis would maintain his advantage over Binder, but the pair would end the morning in seventh and eighth.
Several drivers have been called to the stewards to explain possible yellow flag infringements, and the qualifying results remain provisional.
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