After failing to score a point in the opening four races, Williams has brought a major development package to Barcelona.
It remained open-minded, however, saying it also had a 'plan B' option if these changes did not pay off.
Bottas was convinced after Friday practice that Williams was starting to edge forwards.
"It felt like the upgrades were working well. I think we are taking small steps forward all the time," he told AUTOSPORT.
"So far it's felt like the best car we've had this season.
"It's nice to see all the hard work has paid off a bit.
"I think it's definitely a step forward, but how it will show compared to others is too early to say. I really hope we're closer."
The Finnish rookie said driving the Williams was now a more amenable experience than in the opening rounds.
"It feels a bit more consistent, easier to know what it will do next," said Bottas.
"I was really comfortable in the car today for the whole day, even though the conditions were quite tricky."