As mentioned by The Guardian.
Note: 5 April, 2019 - the tennisdata app is temporarily out of the App Store, undergoing some edits.
In the App StoreTM - http://itunes.apple.com/us/app/tennisdata/id519054852?mt=8
The new version is out. The look of the app has been made much cleaner. The results have a slight color change for every other line to enhance readability.
Under Players, there is now a Rankings option. This will show the player's most current ranking (based on results - if a player has just been playing Challenger or Futures unless they're in the Top 200, their ranking will not be for the current week), his career high ranking, the date he first entered the Top 100 and the date he first entered the Top 10. Also, a Defend Points query; this returns the result and points earned (if any) for the same week last year.
Under Tournament Results, the Full Results for a Year has been upgraded to show the ranking of the players in addition to the seeding. Also under Tournament Results is a Tournament Entrants option. For the selected year, all the players, their seeding and ranking, will be displayed. Like the other data, this is based on current results. This means that the list is being 'built' as the tournament progresses - and won't be complete until all players have played at least 1 match. For tournaments where no seeds receive byes this will be after the 1st round; otherwise after the 2nd.
In the coming months the ranking info will be incorporated into other match results. There will also be enhancements to the tiebreak data.
What is the tennisdata app?
This app is an incredible source of tennis statistics for the ATPTM. Stats by player, tournament, and all up tour stats. Update: as of 21 Feb 2019 there are over 62,000 matches