Kubica has been beaten by his rookie teammate George Russell in every Grand Prix so far. Sirotkin says he'll be ready should Williams come calling.
"I am often asked about this. As a driver, I want to compete with the best drivers in the world in any car, no matter how good it is," Sirotkin told Russian news agency Tass.
Sirotkin is currently Renault's test driver and Cyril Abiteboul said he will do some testing later in the season. He discusses the important role he has on the future of Formula 1.
"My main task is to work on the car for 2021. The car for testing the 18-inch tyres will be ready by the end of September and I will do all the tests with this car. That's my key role," he added.
"Of course, with a team that has such a budget, effort and resources, much more progress and performance were expected [at Renualt]. I'm not going to say what exactly what goes wrong, but when there are no results or progress, the team is under pressure."