You can order by multiple expressions:
ORDER BY games_played < 10, wins / losses DESC
The first expression sorts all those players who have played 10 or more games above all the players that have playes fewer than 10 games. The second expression sorts by win/loss ratio. The second expression is only used to tie-break rows that were equal for the first expression. This means that a player who has played 10 games will always appear above a player who has played only 9 games regardless of their win/loss ratios.