The previous paper of the authors made clear the mechanism of top-spin performance in tennis and its improvement by lubrication of strings according to the high speed video analysis. As the main strings stretch and slide side ways more, the ball is given more spin due to restoring force parallel to the string face when the main strings spring back and the ball is released from the strings. Since the notches of strings decrease spin rate, the lubricant materials are effective to the notched strings and the spin rate of newly strung tennis racket without notches is much larger than that of used racket with notches. Furthermore, it showed that the more spin results in the reduction of shock vibrations of the wrist joint during impact. There are several strings now on the market that are designed explicitly to take advantage of the slippery string mechanism for top spin improvement. This paper showed the spin behavior of the ball and the revolutionary flat strings during oblique impact in the laboratory by the high speed video analysis and recognized that flat strings are more slippery and generate more spin than the conventional strings.