Verstappen won two of the last four races going into the summer break, first in Austria where he performed several impressive overtakes to secure the win and the other in Germany in chaotics weather conditions. His success in recent races has seen him move within seven points of second-placed Valtteri Bottas in the Drivers' Championship.
Despite Red Bull and Honda making big strides this season, Van De Grint feels their fortunes are largely down to the "Max factor" and doesn't feel they really got going at the start.
"We said: what did RBR do? nothing," said Bridgestone's former tire expert in the latest Slipstream from RTL GP.
“It became much stronger later in the season. The chassis has had an influence on this, but so has Honda, which has become much aggressive.
"We look back now and I conclude that they are still not at the Mercedes level. I always call that the Max factor, why they win. If you look at, for example, the second car ... Mercedes still has the lead."