Clark and Mills, who finished runners-up in the 470 women's class at London 2012, were winning before the race was abandoned towards the end.
This means the duo remain fourth in the overall standings and Clark couldn't help but feel frustrated after seeing a victory slip through her fingers.
"We did about 70 minutes of racing and unfortunately we got abandoned right at the end," 34-year-old Clark said.
"We were winning the race at the time, but it is what it is – it doesn't really matter.
"Hannah and I are getting better and are just getting back into the racing. It's really shifty and really patchy conditions [here] which we feel really out of practice in having had the break.
"But I don’t really feel like we’ve done that much sailing in that [condition] anyway in the time that we have had together."
Clark and Mills weren’t the only Brits to feel aggrieved at the venue for the 2014 Sailing World Championships with Luke Patience and Joe Glanfield also seeing no action, although the pair remain atop the 470 men’s standings.
However there was action in the Nacra 17 class with world silver medallist Ben Saxton and Hannah Diamond winning the only race of the day – and the first for the fleet in Spain.
Meanwhile, Nick Dempsey and Bryony Shaw both saw solid starts to their windsurfing events, with world champion Dempsey posting a fourth and Shaw a fifth in the only one of their two scheduled races.