From player ratings, surface, recent results and match context, using a separate model for each tour family. Details are in the methodology.
The model re-runs every hour as new results and schedule data arrive, until the match starts.
A 70% probability means the underdog is expected to win about 3 times in 10.
Only matches the model covers and has enough data for are rated.
See Model performance: how often the favourite won, with the number of predictions.
At the start of the match.