After Tuesday's washout, 10 first round matches were yet to finish, and with all eight second round matches also on the schedule, many players - including Zheng - knew it was going to be a tough one. But the Chinese stepped up, defeating American qualifier Mallory Burdette early on in the day, 62 46 63, then four hours later taking the court again and beating No.1 seed Caroline Wozniacki, 62 64.
The straight set win over Wozniacki wasn't straightforward by any means - Zheng had to stay calm through some tight moments at the end, missing out on her first two match points and watching 62 51 become 62 54 - but she closed it out, firing a huge forehand down the line on match point No.3.
Zheng and Wozniacki had comparable unforced errors in the match - 24 for the Chinese, 21 for the Dane - but Zheng was by far the aggressor, hitting twice as many winners during the match, 34 to 17.
"She's a very tough player and fought until the last point," Zheng said. "I'm so happy I could win this match. We've played many times before and she's always so tough to beat. But she's also a very nice person, ever since she was a junior. I'm very proud and this gives me a lot of confidence."
Zheng, a former World No.15 currently down at No.51, scored the 12th Top 10 win of her career, and the third this year - she had back-to-back wins over Sam Stosur during the Australian season.
Wozniacki wasn't the only big name to fall, with No.3 seed Dominika Cibulkova falling to Kaia Kanepi in a first round match, 64 76(6). Kanepi later beat Jana Cepelova in her second round match, 63 63.
"It was really tough to wake up so early and then play at 10 in the morning!" Kanepi said. "It's tough to play two matches in one day, but I handled it well and I'm happy to have won both matches.
"I felt a little tired in the second match but I really focused and tried not to think about it."
And No.6 seed Julia Goerges also fell, though it was under unfortunate circumstances - having won her first round match earlier in the day, she had to retire after dropping the first set of her second round match against Romina Oprandi, a right wrist injury just too much for her to continue on playing.
Joining Zheng, Kanepi and Oprandi in the quarterfinals were a trio of seeds - No.4 seed Sloane Stephens, No.7 seed Varvara Lepchenko and No.8 seed Peng Shuai. Peng's second round match was the match of the day, as she battled over three hours to edge Olga Govortsova, 46 76(4) 76(4), and also showed steely nerves, fighting back from 4-1 down in the second set and 5-3 down in the third.
Stephens completed her first round match earlier in the day then rolled in her second round match, winning seven straight games from 5-4 in the first set to finish off Zhang Shuai in two, 64 60.
"It's been really tough because the weather's been really poor, but it was good to get on the court today," Stephens said. "I feel good. I feel the clay court season isn't long enough, because I love clay, but I feel good and I'm ready to play. I want to get out there. I'm excited for every match."
Two second round matches will take the court Thursday, as No.2 seed Roberta Vinci and Kazakh qualifier Yulia Putintseva were tied at a set apiece when play was suspended due to darkness, and elsewhere, No.5 seed Kirsten Flipkens and Jamie Hampton never took the court for their match.