The first set took 44 minutes and was a struggle for Williams, who slipped and fell onto her backside while chasing a drop shot while receiving at 5-3.
She got to the ball and saved game point, then closed the set two points later.
The second set took only 27 minutes and Williams was one win closer to her 17th Grand Slam title.
This match was originally scheduled for Wednesday but got pushed back when rain washed out eight women's matches.
Williams has lost only four games in two matches.
In the third round, she'll play Yaroslava Shvedova.
US Open results on Thursday (x denotes seeded player):
Men's singles
Second round:
Richard Gasquet (FRA x8) bt Stephane Robert (FRA) 6-3, 7-5, 7-5
Dmitry Tursunov (RUS x32) bt Guillaume Rufin (FRA) 7-6 (7/4), 1-1, retired, right thigh
Jack Sock (USA) bt Maximo Gonzalez (ARG) 7-6 (7/3), 1-6, 7-5, 6-2
Janko Tipsarevic (SRB x18) bt Dudi Sela (ISR) 6-4, 6-4, 6-1
Mikhail Kukushkin (KAZ) bt Andreas Haider-Maurer (AUT) 6-4, 6-4, 7-5
David Ferrer (ESP x4) bt Roberto Bautista (ESP) 6-3, 6-7 (5/7), 6-1, 6-2
Roger Federer (SUI x7) bt Carlos Berlocq (ARG) 6-3, 6-2, 6-1
Women's singles:
Secoond round:Serena Williams (USA x1) bt Galina Voskoboeva (KAZ) 6-3, 6-0
Yaroslava Shvedova (KAZ) bt Patricia Mayr-Achleitner (AUT) 6-2, 6-3
Kaia Kanepi (EST x25) bt Anna Karolina Schmiedlova (SVK) 6-4, 6-1
Angelique Kerber (GER x8) bt Eugenie Bouchard (CAN) 6-4, 2-6, 6-3
Ekaterina Makarova (RUS x24) bt Bethanie Mattek-Sands (USA) 6-4, 6-4
Sabine Lisicki (GER x16) bt Paula Ormaechea (ARG) 6-2, 6-3
Jelena Jankovic (SRB x9) bt Alisa Kleybanova (RUS) 6-3, 6-2
Kurumi Nara (JPN) bt Sorana Cirstea (ROM x19) 7-5, 6-1
Karin Knapp (ITA) bt Elena Vesnina (RUS x22) 6-1, 6-4
Roberta Vinci (ITA x10) bt Lucie Safarova (CZE) 4-6, 6-1, 6-2
Simona Halep (ROM x21) bt Donna Vekic (CRO) 6-2, 6-1
Svetlana Kuznetsova (RUS x27) bt Peng Shuai (CHN) 6-1, 4-6, 6-4
Flavia Pennetta (ITA) bt Sara Errani (ITA x4) 6-3, 6-1
Petra Kvitova (CZE x7) bt Bojana Jovanovski (SRB) 6-2, 6-4
Alison Riske (USA) bt Mona Barthel (GER x28) 6-4, 6-2
Ana Ivanovic (SRB x13) bt Alexandra Dulgheru (ROM) 6-2, 6-1
Christina McHale (USA) bt Elina Svitolina (UKR) 6-4, 3-6, 7-5
Alize Cornet (FRA x26) bt Ajla Tomljanovic (CRO) 6-2, 6-2