Pliskova, 22, took advantage of a slew of miscues from the eighth-seeded Serb during an 80-minute match in Louis Armstrong Stadium to clinch her most successful Grand Slam appearance.
Ivanovic, never looking comfortable against the punishing, flat groundstrokes and medley of serves launched by the 22-year-old Pliskova, committed 29 unforced errors and seven double faults.
"I really struggled to find my rhythm and made way too many unforced errors," lamented the 26-year-old Serb. "It was just really a bad day."
"It's definitely my best match this year," said Pliskova, who has yet to win on the WTA Tour but who has twice been a runner-up this year.
"I didn't have that much strategy. I just went and played my game. I want to enjoy myself and I did it."
Also shown the door was 2011 champion Sam Stosur of Australia, who lost a third-set tiebreaker 10-8 to Kaia Kanepi of Estonia.
The first meeting between the Americans will not go down as a memorable one, especially for 81st-ranked King as Williams needed a mere 56 minutes to seal the victory on a sun-kissed Arthur Ashe Stadium court.
Williams, dressed in a pink leopard-print dress, had more trouble with the gusty conditions than with her opponent, committing three double faults in her opening service game.
But the 17-times Grand Slam champion would eventually gain enough control of her serve and groundstrokes blasting 25 winners past an overwhelmed King, who could manage just five.
Next up for the top seed in her quest for a third straight US Open title and sixth overall, is American Varvara Lepchenko, who was a 6-4 6-0 winner over Germany's Mona Barthel.
Canada's hopes of a first Grand Slam winner stayed alive as seventh seed Eugenie Bouchard, trying to reach a fourth consecutive Slam semi-final, scored a 6-2 6-7 (4) 6-4 decision over Romania's Sorana Cirstea in the last match to finish on day four of action in New York.
Pennetta dismissed Shelby Rogers 6-4 6-3, while two-times Australian Open winner Azarenka roared back from a 3-love, 0-40 deficit at the start to beat Christina McHale 6-3 6-2.
The Cinderella Grand Slam debut of 15-year-old Catherine Bellis ended in the second round when she was beaten by emerging Kazakh talent Zarina Diyas in three sets.
Bellis became an instant darling of the 2014 US Open by upsetting 12th-seeded Australian Open finalist Dominika Cibulkova of Slovakia, making her the youngest player to win a match at Flushing Meadows in 18 years since Anna Kournikova.
Fans packed Court 17 and lines of spectators waited at every entrance to the cozy 2,800-seat stadium on Thursday for a chance to cheer on the pony-tailed Bellis.
The mega screen outside Arthur Ashe Stadium by the fountains in the main plaza was trained on the outer court drama while former US Open and Wimbledon champion Andy Murray battled on the centre court.
In the end, hopeful vibes from the local crowd at the US National Tennis Center could not change the script even as the stands rocked with chants of "CiCi" - the Californian teenager's nickname.
Bellis put up a valiant fight, winning seven games in a row after dropping the opening set, but the 20-year-old Diyas proved too strong and too consistent and registered a 6-3 0-6 6-2 win.
US Open results
7-Eugenie Bouchard (Canada) beat Sorana Cirstea (Romania) 6-2 6-7(4) 6-4
17-Ekaterina Makarova (Russia) beat Polona Hercog (Slovenia) 6-1 6-2
Kaia Kanepi (Estonia) beat 24-Samantha Stosur (Australia) 3-6 6-3 7-6(8)
Aleksandra Krunic (Serbia) beat 27-Madison Keys (U.S.) 7-6(4) 2-6 7-5
3-Petra Kvitova (Czech Republic) beat Petra Cetkovska (Czech Republic) 6-4 6-2
15-Carla Suarez Navarro (Spain) beat Coco Vandeweghe (U.S.) 6-3 6-3
Nicole Gibbs (U.S.) beat 23-Anastasia Pavlyuchenkova (Russia) 6-4 6-7(6) 6-3
30-Barbora Zahlavova Strycova (Czech Republic) beat Monica Niculescu (Romania) 4-6 6-4 6-2
Elena Vesnina (Russia) beat Marina Erakovic (New Zealand) 7-5 2-6 6-4
Varvara Lepchenko (U.S.) beat Mona Barthel (Germany) 6-4 6-0
1-Serena Williams (U.S.) beat Vania King (U.S.) 6-1 6-0
16-Victoria Azarenka (Belarus) beat Christina Mchale (U.S.) 6-3 6-2
29-Casey Dellacqua (Australia) beat Wang Qiang (China) 4-6 6-4 6-2
Karolina Pliskova (Czech Republic) beat 8-Ana Ivanovic (Serbia) 7-5 6-4
11-Flavia Pennetta (Italy) beat Shelby Rogers (U.S.) 6-4 6-3
Zarina Diyas (Kazakhstan) beat Catherine Bellis (U.S.) 6-3 0-6 6-2
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