Williams' path may have been eased by Kaia Kanepi's stunning upset of world number one Halep, who became the first top-seeded woman to lose in the first round of the US Open. Making her 11th main draw appearance in Flushing Meadows, a year ago Kanepi completed the best result by a qualifier at U.S.
No.7 seed Elina Svitolina powered into the second round of the US Open thanks to a 6-3, 1-6, 6-0 victory over Sachia Vickery who broke in the very first game before Svitolina claimed five straight games.
"I have always loved being in NY", said the 33-year-old Kanepi, twice a quarter-finalist at the tournament who has been ranked as high as world number 15.
Despite battling back to 4-4 in the second set, Halep wasn't able to hold on.
Halep was off to another slow start at a Grand Slam tournament.
Ashleigh Barty has made a confident start to her US Open campaign with a crushing first-round win over Tunisian qualifier Ons Jabeur. Kanepi said after the match that her key to success was "staying aggressive all the time", which paid off as she was able to repeatedly break Halep's serve.
"I never play my best tennis here, even if I've done quarterfinals, semifinals", Halep told wtatennis.com. Monday's victory was Kanepi's first win over a top-20 player since 2015.
"I feel pretty good", Williams said.
"She has been playing really well in U.S. lately, so I thought I just have to be aggressive and try to stay calm". "But also, she played really strong and pushed me back, so it was tough". "It was a bit annoying for some time, but I got over it".