French Open champion through to third round in three sets; Radwanska, Stephens sent crashing out at Flushing Meadows.
Maria Sharapova fought back to avoid an early exit on a sweltering day at the US Open after fourth-seed Agnieszka Radwanska was dumped out.
French Open champion Sharapova lost the first set to 95th-ranked Alexandra Dulgheru of Romania, but the five-time grand slam winner bounced back to reach the third round with a 4-6, 6-3, 6-2 victory in warm breezes at Arthur Ashe Stadium.
Radwanska was not so fortunate as she fell 6-3, 6-4 to world number one doubles player Peng, who supplied a shock start to second-round play in Louis Armstrong Stadium.
World number two Simona Halep of Romania beat the heat by hurrying past Jana Cepelova of Slovakia 6-2, 6-1 to reach the third round.
Johanna Larsson of Sweden, ranked 96th, sent another seed to the exits with a 5-7, 6-4, 6-2 upset of 21st-seeded American Sloane Stephens, a 2013 Australian Open semi-finalist.
Meanwhile, Venus Williams kept up with little sister Serena, beating 78th-ranked Timea Bacsinszky of Switzerland in straight sets to advance to the third round.
The 34-year-old, seven-time grand slam winner reached the round for the first time in four years at Flushing Meadows through a 6-1, 6-4 victory on the Arthur Ashe Stadium court which she helped christen with the first Open final there in 1997.
Others advancing included sixth seed Angelique Kerber of Germany, ninth-seeded Serbian Jelena Jankovic and 10th-seeded former world number one Caroline Wozniacki of Denmark.