Sloane Stephens had to dig deep into her reserves to keep her US Open defence on track, while the latest instalment of the Williams sisters' rivalry remains on track.
On another sweltering day at Flushing Meadows, Stephens felt the heat before bouncing back to get over qualifier Anhelina Kalinina at Arthur Ashe Stadium.
Venus Williams had a somewhat easier progression against Camila Giorgi, and now faces the possibility of a third-round clash with Serena.
VENUS SETS UP POTENTIAL 30TH SERENA CLASH
There have been 29 previous competitive clashes between the dominant Williams sisters, with Serena winning 17 to Venus' 12.
Elder sibling Venus, who defeated Serena at Indian Wells earlier this year, may have the chance to close the gap after beating Giorgi 6-4 7-5.
"The last time we played the Australian it was two against one so at least this time it will be fair," Venus joked in reference to the fact Serena was pregnant when they faced off at the 2017 Australian Open.
"I hope that we get to play. I'll do everything I can to recover and play in this tournament."
SUSHI BREAK HELPS SLOANE BOOK AZARENKA DATE
World number three Stephens toiled in the searing New York heat, but was able to take a 10-minute break in her gruelling 4-6 7-5 6-2 victory over the unfancied Kalinina.
The defending champion later said she had "two bites of sushi and a slushy" during the heat break.
"It was super hot," she said. "I was sweating a lot and it was not ideal conditions, but I am happy to get through."
Victoria Azarenka should provide another stern test in round three, with the former world number one outclassing Daria Gavrilova 6-1 6-2.
SVITOLINA, PLISKOVA UNTROUBLED
Elina Svitolina, the seventh seed, cruised into the third round thanks to a 6-2 6-3 win over Tatjana Maria.
Karolina Pliskova, the 2016 runner-up, was also untroubled in a 6-2 6-3 victory against Ana Bogdan.