As observed from the earth, the sun appears to move in an approximate circular orbit. For the motion of another planet like mercury as observed from the earth, this would

As observed from the earth, the sun appears to move in an approximate circular orbit. The gravitational force of attraction between the earth and the sun always follows inverse square law.
Due to relative motion between the earth and mercury, the orbit of mercury as observed from the earth, will not be approximately circular, since the major gravitational force on mercury is due to the sun.