Today, hugging has become the new handshake, providing a level of intimacy that may make postmatch meetings at the net as unrecognizable to past players as innovations like the Hawk-Eye ball tracking system and serving time limits.
World number 88 Petkovic was the better player in the biggest points, breaking the twice Grand Slam champion three times and committing 15 fewer unforced errors on a sun-soaked Louis Armstrong Stadium court.
Naomi Osaka was given a testing workout by Anna Blinkova as she began the defence of her US Open title, while Wimbledon champion Simona Halep snapped a string of early US Open exits with a 6-3 3-6 6-2 victory over Nicole Gibbs to reach the second round.
Naomi Osaka has gone on to become the second-highest-paid female athlete in the world, behind Serena Williams, off the back of a host of lucrative sponsorship deals. But with the tennis world at her feet, the 21-year-old struggled to kick on.
Naomi Osaka began retired from her quarterfinal match at the Western & Southern Open, unsure what's going on in the knee. The defending U.S. Open champion is worried about her chances of making another deep run in New York.
Osaka, who won the Australian Open in January to follow on from her dramatic victory over Serena Williams at the U.S. Open in 2018, is looking to become just the 11th woman to claim a third consecutive Grand Slam title.