Serena Williams and Naomi Osaka survived tests at the Western & Southern Open, while Madison Keys and Petra Kvitova exited.
Williams, the third seed at the WTA Premier tournament, progressed after close to three hours on court in New York on Monday.
Osaka was also pushed before progressing to the last 16 at the event, which is usually held in Cincinnati.
However, Keys and Kvitova were among the departures.
SERENA SURVIVES TEST
Williams was taken to the brink before prevailing 7-6 (8-6) 3-6 7-6 (7-0) against Arantxa Rus after two hours, 49 minutes.
A 23-time grand slam champion, Williams served 14 aces – including four in the final set – but was pushed by Rus.
Williams served for the match at 5-3 before losing three straight games, Rus also unable to serve it out before the American advanced.
Awaiting Williams, a two-time winner of the event, in the last 16 is Maria Sakkari after the Greek 13th seed beat Yulia Putintseva 6-4 7-6 (11-9).
OSAKA PUSHES THROUGH
Osaka was also challenged before the Japanese fourth seed edged Karolina Muchova 6-7 (5-7) 6-4 6-2.
After levelling the match, Osaka raced into a 4-0 lead in the deciding set on her way to victory.
Osaka will next face 16th seed Dayana Yastremska, who recorded a 7-6 (7-5) 4-6 6-3 win over Bernarda Pera.
Aryna Sabalenka (five), Johanna Konta (eight), Anett Kontaveit (12) and Elise Mertens (14) were the other seeds to advance.
KEYS' TITLE DEFENCE OVER
Defending champion Keys slumped to defeat in the opening match of her title defence, going down to Ons Jabeur 6-4 6-1.
Keys, the seventh seed, won just 10 points against the Jabeur serve, with the Tunisian managing four breaks of her own.
Kvitova was the other seed to fall, losing an all-Czech battle against Marie Bouzkova 2-6 7-5 6-2.
Meanwhile, 2013 champion Victoria Azarenka progressed courtesy of a 6-2 7-6 (10-8) win over Caroline Garcia.