The world number one showed no signs of the virus which curtailed her appearance at the All England Club in June as she downed Czech Pliskova 7-5 6-2 in front of a supportive crowd at the Bank of the West Classic in California.
American Williams, the top seed, was pushed hard in the first set before she opened up in the second to seal victory in 64 minutes.
"I thought I played well," Williams said. "I'm on the right path, I'm excited and I'm looking forward to my next match."
Earlier in the day, second seed Angieszka Radwanska was stunned by Varvara Lepchenko, the American who is ranked 59 in the world.
Radwanska reached the final in Stanford 12 months ago, but the Poles' hopes were ended in a 6-3 3-6 6-4 victory for Lepchenko, who beat a top-10 ranked player for just the third time in 24 attempts.
Lepchenko moves through to a quarter-final clash with American qualifier Sachia Vickery, who beat Monica Puig 6-7(4) 6-2 6-1.
Meanwhile, Andrea Petkovic cruised into the last eight with a regulation 6-2 6-2 win over Naomi Osaka, the 16-year-old Japanese who upset Sam Stosur in the first round.
Stanford Classic results
1-Serena Williams (U.S.) beat Karolina Pliskova (Czech Republic) 7-5 6-2
8-Andrea Petkovic (Germany) beat Naomi Osaka (Japan) 6-2 6-2
Varvara Lepchenko (U.S.) beat 2-Agnieszka Radwanska (Poland) 6-3 3-6 6-4
Sachia Vickery (U.S.) beat Monica Puig (Puerto Rico) 6-7(4) 6-2 6-1
- Sports & Recreation
- Stanford University
- Varvara Lepchenko