orderByAscending header is not supported for this method yet. It is under development and will take few weeks to hit the production.
Alternatively, you can save the remaining time instead of played time. For example if you have time out in your game of 300 seconds, and if user has taken 30 seconds to complete, you can save 270 seconds as remaining time in leaderboard. In this case the user who has highest remaining time would be a top ranker in default leaderboard.
Let me know if it helps in solving your query.