Williams double faulted on the first point of the match and lost serve at love in that opening service game, then watched as Barthel held routinely to take a 2-0 lead. But Williams came alive from there, reeling off 12 of the next 14 games to completely dominate the German in just over an hour, 63 61. She finished the match with 20 winners to just 13 unforced errors - Barthel was 10 winners, 22 unforced.
"I haven't had the health to be here for a long time, so I'm very excited to be back here in Tokyo," Williams said. "I was waiting to come here since the US Open and I'm happy to win my first match."
Next up for Williams is a mouthwatering second round clash with the No.1 seed, Victoria Azarenka.
"I've watched so many big matches between her and Serena it seems like, and I just watched them at the Open, which was awesome," Williams said. "Hopefully that'll help me, but I've played so many matches in my life, so if I play another top player I have experience and know I just have to play well."
Williams has beaten Azarenka in both previous meetings, though they haven't played since 2010.
"It's my first time in Tokyo and I'm excited to be here," Stephens said. "I couldn't really see well at the beginning of the match because the lights were pretty bright, but I thought I played pretty solid!
"I'm playing my first official fall season, so I look forward to this week and the rest of the fall."
Stephens was also asked about her vibrant on-court fashion during her post-match press conference.
"I think I'm going to finish the year in purple," the American said. "I get some input into my outfits; I pick all the colors at the beginning and then Under Armour puts them with whatever outfit they want to.
"We work a long way in advance. We're already planning 2015 outfits. The outfits in 2014 are going to have more bright colors and they'll be really cute. They're taking it a bit more fashion forward."
The other seeds in action all advanced too, with No.12 seed Sam Stosur breaking away from 4-all in the second set to beat Alizé Cornet, 26 64 61; No.14 seed Kirsten Flipkens saving set point late in the first set and beating Czech qualifier Barbora Zahlavova Strycova, 76(4) 75; No.15 seed Sorana Cirstea beating Julia Goerges, 64 64; and No.16 seed Dominika Cibulkova beating Urszula Radwanska, 63 63.