Nara, ranked No. 238, reached the main draw at Roland Garros via the qualifying tournament. The gulf between her and Williams, a 23-time major champion, was quickly apparent on Court Philippe Chatrier.
Williams faced no break points, served 10 aces and made 36 winners as she improved to 24-0 against qualifiers in Grand Slam events -- the most wins without a loss by any woman against qualifiers since the start of the Open Era.
She will next play Sofia Kenin, ranked No. 35. Williams is 39-2 against younger American players during her tour-level career.
The Associated Press contributed to this report.