It didn't help that one of his biggest weapons was off, as Venus managed only 43 per cent of first serves.
It resulted in the Kiwi being broken five times, while Bautista-Agut fended off five of the six break opportunities that Venus created.
"I couldn't get going with my serve and forehand so when you are not on the first foot from those service points, it's tough," said Venus.
"I played some good points but that was followed by some loose ones. That was all part of the pressure. The guy doesn't give you anything and to win points it felt like I had to hit three or four balls that with some guys you might win the point on one of those. It all builds and you start missing."
But Venus will play alongside his new partner Raven Klaasen in the first round of the doubles on Tuesday night, against compatriot Artem Sitak and Wesley Koolhof (Holland).
The Classic lost a few of its young guns on Monday, though one highly touted ATP Next Generation name impressed in his first outing.
Eighth seed Andrey Rublev withdrew from the tournament shortly after arriving yesterday, citing an arm injury.
Then last night British youngster Kyle Edmund also pulled out, with a right ankle sprain.
But Canadian Denis Shapovalov lived up to all the hype, with a free wheeling 6-3 6-2 win over Brazilian qualifier Rogerio Dutra Silva that featured some brilliant shot making.