I wouldn't chance it. Get AirAsia to change to morning flight, or cancel/refund&buy new flight
https://www.flightstats.com/v2/fligh...ing/QZ/508/DPS
Ontime performance wise is okay (avg 20min delay max 3 hr). Even if you assume that 8:15pm+20min = 8:35pm. You land at T4, have to travel to T2 (transit security, 10-20min bus commute, then walk to gate). UA doesn't have mobile passes for SIN- you may have to visit the transit desk to get pass, or get it at gate.
If you have to exit immigration and re-enter, additional time. More time for bags too
There may be additional medical screening measures that take time
UA does have an unofficial flat tire rule, but you really don't want to rely on it