The 36-year-old Williams raced out to a 3-0 lead but Barthel fought back with the American having trouble with her ball toss at the northern end of the court as she looked directly into the sun.
Barthel broke twice to give her the opportunity at 4-3 to get back on serve but Williams capitalised on the German also having trouble with the sun to break back then served out from the Yarra River end to clinch the first set in 42 minutes.
