Lob :
I did a try with a defender CPU always lobbing, in Pro level, with 100% in speed and 1m85 high.
With 100% in all forehand skills, I could win by winners about 25% of points, and ~10% of faults (coz he was running too much when hitting the lob). I guess with a human the result would have been a bit better, probably near of 50%, especially coz the CPU knows his own speed very clearly and acts more or less accordingly. To do the winners, I had to stand just after the baseline, about 30cm behind.
I also tried to rush the net after the start of the lob, and I could almost reach the service line where I could do an accelerated volley with a good success ratio.
Anyway, it makes the gameplay very painful to have the opponent doing only lob, so I'll try to raise the acceleration speed by ~5km/h on high balls for next update, we'll see if that helps...
Lob on return:
Normally, it should be hard to lob on return, coz it needs to stand very very far of the baseline to avoid the "over powered" effect (when u can't control the incoming ball speed, and u send the ball too long). When the returner stands so far of the baseline, it should give time to the server to rush the net when he sees he's getting a lob on return, no ? (this is less true when the lag is > 50ms, coz in this case, the service power is limited to 60%, but the returner still has to stand a bit far of the baseline)
TopSpin :
Right now, I don't have idea to balance that, except the additional 5km/h on high ball should help it a bit as well...
Is the problem only with 100% Topsin + Defender ? or any style + 100% Topspin is too annoying ? If it's only with the Defender, I could lower a bit its anti-length handicap when doing the topspin strike.
Slice :
the slice animation has no effect on the strike when doing a acceleration (except it changes the sweet spot position).
For next update, the slice speed will also be increased by up to 10km/h, depending of the topspin skill. The higher the skill, the lower the speed bonus. It'll allow to do lower slice (thus, it might help better to fight the topspin).
Player Speed:
The max speed is 30km/h when u have 100% in Speed, whatever ur height is.
For a tennis pro, this is very fast, but it's far to be inhumanely fast, coz top sprinters can reach ~40km/h (but without holding a racket

To run from left lane line to right lane line takes ~0.05s less for a 1m55 height player compared to a 2m player. That's not exactly a lot faster...

The "huge" difference of speed perceived is mainly due to the difference in player size ; ie: this is an optical illusion due to the ratio of speed / size. This ratio is 30% bigger for the 2m high player, thus the perceived relative speed is 30% smaller. And his legs also moves 30% slower, reinforcing the optical illusion...

@Tom Paulman :
>> Fair Mode: Service max. 80 %
This limit is only for the power.
@S.Williamz :
>> when your opponent uses a lot of topspin, every time you are hitting, you are pushed automatically a lot back
By "automatically" u mean u are forced to step back by urself, or that the auto-pos makes u walk back ? The auto-pos being on medium in Online games, I assume u mean u have to walk back by urself...