Williams chief technical director says Mercedes has "chinks in their armour" and can be beaten in the 2016 season.
Mercedes has been dominant in the V6 turbo era, winning all but six of the races in two years and claiming back-to-back drivers' and constructors' championships. Williams has been a resurgent force since switching to Mercedes power in 2014 but is yet to return to the top step of the podium.
Symonds is adamant Mercedes will not be unbeatable in the upcoming season and does not think the current situation compares to other dominant teams in history.
"Even if you get as dominant a situation as Mercedes have got, we are not in the McLaren 1988 era when they absolutely win every race [with Ayrton Senna and Alain Prost]," Symonds told UK news agency Reuters."Mercedes have chinks in their armour. There aren't many because they are an amazingly strong team but we've got to be there for those days.
"At the same time we have to keep pushing ourselves so we are not [just] looking for that way of doing it, ultimately what we want to do is beat them fair and square. That's what we've got to aspire to."
Williams led a one-two in the early stages of last year's British Grand Prix but failed to convert its strong position into a win or podium finish for either driver. Symonds knows Williams has to operate better at every level this season if it wants to go a step further.
"We certainly made far too many operational errors last year, there's no doubt about it. In my pre-season briefing to the guys at the beginning of the year we concentrated a lot on that.
"I can't say we will be faultless, nobody is immune... but certainly the number of errors we made was way too high and we are working hard to try and improve that."