First of all, congratulate Manu on his job on this game, not only the best tennis game but a great career mode.
Schedules in tennis are designed by surface, having tours that last several weeks on a specific surface that end with the corresponding Grand Slam (in the case of clay and grass). Players adapt their schedules and training routines in order to get to their target tournament, normally the Grand Slam, in top shape and fully adapted to the surface. That part of preparing the shedule thinking in the future tournaments is missing in career mode, and could add additional depth to the seasons.
Maybe there could be 3 stats for adaptation, one for each surface, improved not only by playing games on that surface, but also by training. Independent of the skill points you can distribute among the skills, there could be "surface points" or something like that that would represent the surface you are training on. That way, you don't need to play every single tournament of clay or grass in order to reach a good level of adaptation, but playing the tournaments would improve the stat quicker than training can.
How the adaptation levels affect the stats could be done in different ways, but probably a very small nerf to some or all skills would be enough to incentivize training surfaces without being too much of a hassle.



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