EXCLUSIVE

R. Kelly's accuser has run into a huge roadblock in her lawsuit against the singer ... so she's running to the judge for some help.

According to new legal docs obtained by TMZ ... Heather Williams says she's desperately tried to serve Kelly with legal papers but claims security is so tight at Trump Tower where Kelly lives, it's proven to be mission impossible.

In the docs, Williams says she even tried giving a copy to Cook County Sheriff's officials but says security at Trump Tower would not let deputies past the lobby. She's filed a motion to get the judge to allow her to serve the papers on Kelly's lawyers instead.