Williams needed just 1 hour, 4 minutes at Ariake Colosseum to dispatch Barthel and set up a second-round matchup with top-seeded Victoria Azarenka, who has a first-round bye.
Top-ranked Serena Williams, who won her 17th Grand Slam singles title at the U.S. Open, pulled out of the $2.3 million tournament, saying she needed time to recover from a busy summer playing schedule.
No. 3-ranked Maria Sharapova also pulled out of the Tokyo tournament because of a right-shoulder injury that kept her out of the U.S. Open.
In other matches, ninth-seeded Sloane Stephens of the United States beat Stefanie Voegele of Switzerland 6-3, 6-2 while 12th-seeded Samantha Stosur of Australia came from a set down for a 2-6, 6-4, 6-1 win over Alize Cornet of France.
Kirsten Flipkens of Belgium, seeded 14th, defeated Barbora Zahlavova Strycova of the Czech Republic, 7-6 (4), 7-5.
Sorana Cirstea of Romania beat Julia Goerges of Germany, 6-4, 6-4.