Riggs was certainly a touch player; but through disguise, essentially, he got a lot of aces. More than players who served harder than he did.
This is what Vines wrote:
His balanced, penetrating groundstrokes on both sides were at least the equal of Rosewall's, including return of serve. Speed never disturbed him, not even a big serve. Budge had a harder delivery, yet Bobby's service was probably more effective because he had more varieties. He could hit a cannonball, slice, or American twist. The only server that gave Riggs real trouble was Kramer, because of Jack's unique ability to spot and mix up deliveries.
At one point during the ’42 pro tour Riggs was out-acing Budge, Perry and Kovacs, per a press report:
Riggs, whose habitual slow-hook serve contrasts strangely with the express-train deliveries of his companions, regularly serves far more aces than any one of them in the matches of the tour. Reason, Bobby draws his opponents out of position and lulls them into false security, then slips quick straight ones down the opposite side of the service court for 'surprises' that leave opponents flat footed.
Vines wrote that Riggs, after the war, was out-acing Budge, Kramer and Gonzalez.
Compare these ace counts for Bobby and Pancho in matches at Forest Hills.
Riggs in '48 pro semi -- 1.3 aces per game (d. Kovacs w/ 20 aces))
Riggs in '49 pro final -- 0.7 aces per game (d. Budge w/ 17 aces)
Pancho in '48 amateur final -- 0.8 aces per game (d. Sturgess w/ 16 aces)
Pancho in '49 amateur final -- 0.8 aces per game (d. Schroeder w/ 27 aces)
The New York Times said that Riggs' serving performance in that first match in '48 was "one of the most remarkable exhibitions of serving" that they had ever seen. "With comparatively little effort, Riggs gets remarkable speed on the ball, but it is more the spin and the cleverness and accuracy with which he places the serve that makes it so difficult to return. It was nothing less than demoralizing."
Ironically Kovacs, who was 6'4", served just 3 aces.