Before you start play, on your IPhone, make sure your Watch App is set so that Watchpoint stays active during your match. Open the watch app on your Iphone, go to General->Wake Screen and check "Within 1 Hour of Last Use" on "On Screen Wake Show Last App." Also, set it to "Wake for 70 Seconds." Now you're ready to use WatchPoint.
This will keep the app running while your're playing your match. After the match, you can return it to whatever settings you choose, but remember to reset it before you use WatchPoint again!