There are so many different kinds of pressure that tennis players can exert on their opponents over the course of a match.
Blasting massive serve after massive serve. Hitting deep. Hounding the baseline to push the opponent on the other side of the net into the back of the court. Rushing the net and standing tall there, unafraid, just 35 feet away. There is even the pressure of the scoreboard that comes with early leads in games, or of the softest drop shots that can land like uppercuts to the gut.
An ability to get a crowd of more than 20,000 to raise the decibel count to uncomfortable levels at the crucial moments also helps.