Serena Williams began her pursuit of a record-equalling 24th Grand Slam singles title with a dominant 7-5 6-3 win over fellow American Kristie Ahn.
The 38-year-old went a break down in both sets but recovered to win in an hour and a half as she chases Margaret Court’s Grand Slam tally.
Williams gathered momentum late on, winning four straight games against the world number 96 in the second set.
“I was really happy with how I fought for every point,” she said.
“No matter how I was playing, I just had to get my Serena focus back so that was what I was working on today.”
But she will not be joined in the next round by her sister Venus Williams, who lost 6-3 7-5 to Czech 20th seed Karolina Muchova.
An error-strewn performance meant that two-time champion Venus, 40, made her first ever opening-round exit at her home Grand Slam.
Women’s 10th seed Garbine Muguruza sealed her place in the second round with a 6-4 6-4 win against Japan’s Nao Hibino.
Seventh seed Madison Keys is through after beating Hungary’s Timea Babos 6-1 6-1, while American second seed Sofia Kenin claimed a 6-2 6-2 victory against Belgium’s Yanina Wickmayer.
But former world number one Kim Clijsters’ return to Grand Slam tennis ended in defeat as Ekaterina Alexandrova of Russia prevailed 3-6 7-5 6-1.
Belgium’s Clijsters – a three-time US Open champion – was given a wildcard for the tournament after coming out of retirement earlier this year.