Serena Williams admits she was far from at her best in Wednesday’s US Open second round win over Kiki Bertens.

The American arrived at Flushing Meadows looking to become the first woman since Steffi Graff in 1988 to win the calendar Grand Slam and had little difficulty in her opening match when her opponent Vitalia Diatchenko retired through injury.

However, she was firmly tested by Bertens, with the Dutchwoman at one stage leading 4-0 in the first set tie-break, before Williams fought back to claim a 7-6 6-3 victory at Arthur Ashe Stadium.

Next up for the 33-year-old is wildcard Bethanie Mattek-Sands, a match she is 1.03 to win and her opponent the 11.00 outsider, as she plots her way towards a potential 22nd Grand Slam title.

Williams is currently the clear 1.73 favourite to win at Flushing Meadows, with Victoria Azarenka second in the betting at 8.00 and Simona Halep priced at 10.00.

Despite the pressure on her coming into the final major of the year, the world number one has largely managed to remain relaxed throughout her preparations.

The 33-year-old admits she struggled to find her natural rhythm on Wednesday and concedes she must find a way to get back to her usual level if she is to claim her national title for the fourth year in a row.

“I just kept fighting for each point, not for a lot but just one at a time,” Williams said. “I had been pretty relaxed. Today I was a little tight.

“I think it showed. Hopefully I can get back to where I was before.”