NEW YORK: It's rare that a world number one and defending champion is the warm up act at the U.S. Open but that was the case on Monday as Daniil Medvedev played second fiddle to Serena Williams, who takes to the court later having hinted the match could be her last.

Williams, 40, said in a Vogue article in early August that she was "evolving away from tennis" but has never confirmed the U.S. Open as her final event.

For tennis fans, however, the message was clear - the U.S. Open would be where the 23-times Grand Slam winner would take her final bow.

Eager to witness what may well be Williams' final competitive match, thousands of fans poured into the Billie Jean King Tennis Center on Monday for her early evening clash against Montenegro's Danka Kovinic.

After the Day One schedule was released with Williams slotted in for 7pm local time start, ticket resale site StubHub said it saw and immediate spike in demand for Monday's evening session.

The first two days of play at Flushing Meadows are commanding a higher average price of tickets sold ($224) than the opening two days in 2021 ($157) but a courtside seats for Williams opener were going for $1600.

"With excitement building around Serena Williams’ first-round match-up against Danka Kovinic, we’re seeing more than three times the demand for the first day of the U.S. Open in comparison to day one last year,” Adam Budelli, spokesperson for StubHub told Reuters in an email.

The opening day at any Grand Slam comes with an identifiable buzz with action underway on every court and cash registers humming.

But across the sprawling grounds of the Billie Jean King Tennis Center that buzz was amplified and growing as giddy fans looked forward to seeing Williams, widely considered the greatest woman's player ever.

Around the Baseline Bar outside Arthur Ashe Stadium court, the chatter centred around only one topic, with few taking notice of the match between Medvedev and American Stefan Kozlov, playing on the jumbo-tron.

"Is this really her (Williams) last match? I don't believe it," questioned one tennis fan sipping on a $22 Honey Deuce cocktail, sparking a debate that carried on until another round was ordered.

All will be revealed later but retirement will not be clear cut.

A loss to the 80th ranked Kovinic will not be end of Williams career or time at Flushing Meadows, with the former-world number one also scheduled to play doubles at the season's final major with older sister Venus.