Former Wimbledon champions Angelique Kerber and Petra Kvitova kept their US Open hopes on track after gritty second-round wins.
Kerber, the 17th seed, is playing in her 50th grand slam at the venue where she made her breakthrough, reaching the semi-final in 2011.
The 2016 Flushing Meadows winner came through a tight all-German clash against Anna-Lena Friedsam 6-4 7-6 (6).
Kerber, 32, who won Wimbledon two years ago, said: "It's always special to return to a place where you won. It's a really special place for me, where it all started in 2011, and I'm still here competing. This is what I love."
Kvitova, the two-time champion at SW19, has never made it further than the quarter-finals in New York. The 30-year-old Czech, seeded six at this year's championship, was slow to get moving but eventually beat Ukrainian Kateryna Kozlova 7-6 (3) 6-2.
She said: "It was very difficult especially with my nerves, I felt very tight. But it's a grand slam, at the beginning of the tournament it's always like this."
Serena Williams vowed to relish every moment of her quest for a record-equalling 24th grand slam singles title after edging her way into the second round.
The 38-year-old, who needs one more trophy to match Margaret Court's all-time record, was forced to hit back from an early break down in both sets before seeing off American compatriot Kristie Ahn.
In doing so, six-time champion Williams won her 102nd career match at the US Open, surpassing the previous record she shared with Chris Evert.
Even with six of the world's top 10 players absent, Williams' 7-5 6-3 win did not exactly inspire confidence that Flushing Meadows will provide that historic platform in little under two weeks' time.
But Williams joked: "It's been since the 1990s that I won a match in straight sets so that feels really good. I just told myself, 'be Serena and close it out', and I know I can do that.
"I love my job at the end of the day and I've always said that you can't do it forever. One of these days it is going to end and I just love being out here."
Williams has endured a below-par run of form either side of the lockdown and came into the tournament on the back of a heavy loss to Maria Sakkari in the Western & Southern Open at the same venue last week.