Add the spaghettini to the boiling water. When the pasta is very al dente, remove it with tongs and add directly to the bowl with the tomatoes, reserving the pasta water. Toss well and let sit a minute or two for the tomato juices to soak into the pasta, then add a little more pasta water if the pasta seems dry. Sprinkle with the grated Grana Padano, toss, and serve.