The sense of smell in a tiger is at its lowest. As we are all aware, tigers spray scent to mark their territory. A study involving chemical analysis of the scent marking by tigers had revealed that there are 45 different chemicals in the scent mark of a tiger and the composition varies according to the mood or message to be conveyed. (I don't remember the name of the scientist and am quoting from memory)
In this image, we can see that the tiger has curled its upper lip to smell the scent left behind by another tiger. Curling of the upper lip in this manner helps in transferring pheromones and other scents into the Jacobson's organ or vomeronasal organ. This behaviour can also be seen in ungulates as well.
This curling of the upper lip is known as Flehmen response.