  Identifying key patterns of tactics implemented by rival teams, and
developing effective responses, lies at the heart of modern football. However,
doing so algorithmically remains an open research challenge. To address this
unmet need, we propose TacticAI, an AI football tactics assistant developed and
evaluated in close collaboration with domain experts from Liverpool FC. We
focus on analysing corner kicks, as they offer coaches the most direct
opportunities for interventions and improvements. TacticAI incorporates both a
predictive and a generative component, allowing the coaches to effectively
sample and explore alternative player setups for each corner kick routine and
to select those with the highest predicted likelihood of success. We validate
TacticAI on a number of relevant benchmark tasks: predicting receivers and shot
attempts and recommending player position adjustments. The utility of TacticAI
is validated by a qualitative study conducted with football domain experts at
Liverpool FC. We show that TacticAI's model suggestions are not only
indistinguishable from real tactics, but also favoured over existing tactics
90% of the time, and that TacticAI offers an effective corner kick retrieval
system. TacticAI achieves these results despite the limited availability of
gold-standard data, achieving data efficiency through geometric deep learning.
