I hope the following pictures can resolve this dispute.
Figure 6. Arm and racquet motions around contact
The fig.6 demonstrates that tip of the racquet moves upward and to the right. The frame #4 is contact point. The arm pronation, wrist flexion, arm and body forward rotation move the racquet to the right and create flat component V
F of the racquet velocity V
R, see fig.7. The wrist ulnar deviation and elbow flexion move the racquet upward and produce topspin component V
Topspin. I ignored horizontal spins, because they are not essential here.
Figure 7. Ball’s trajectory after impact
So, velocity of the racquet V
R is facing up and to the right, not downward. The flat component V
F is directed downward and to the right.
However, why the direction of the ball V
B does not coincide with racquet velocity direction V
R? The fact is that some of the energy of V
Topspin is spent on topspin production.
About vectors and their components see http://tt.tennis-warehouse.com/showthread.php?t=436086