I resolved this issue by myself. The root cause is that version of my SupportAssist OS Recovery tool is too low. What I did: (1) Forced my desktop to restart when it was stuck at "System Configuration" stage which was indicated as 85% complete. (2) Connected my desktop to internet via cable and re-entered SupportAssist OS Recovery tool. (3) SupportAssist OS Recovery tool automatically found latest version online and prompted with an upgrade button on top of the page. (4) Restored OS successfully after upgrading SupportAssist OS Recovery tool to latest version.  
I have an alienware aurora R9 running windows 10. Today when I try to recover OS by SupportAssist OS Recovery tool, the progress stuck on the page called "Alienware Image Customization". All items on this page were completed with green ticks in front except one called "System Configuration". The progress is 85%. May I ask what should I do now? Should I keep waiting? Or should I restart the desktop?
SupportAssist OS Recovery Stuck at System Configuration