Venus Williams was the highest-profile casualty at the WTA China Open on Tuesday as the top three seeds all claimed victory.
World number one Angelique Kerber, reigning champion Garbine Muguruza and Agnieszka Radwanska recorded straight-sets wins in Beijing.
Yet Williams suffered a shock 7-5 6-1 defeat to Chinese wildcard Peng Shuai.
VENUS VANQUISHED DESPITE STRONG START
Sixth seed Williams led 4-2 in the first set of her match with Peng, the world number 223.
However, a surprising shift in fortunes followed, Peng dropping just two games thereafter as she saved two set points in the opener before racing to victory.
A former top-20 player who has been blighted by injury in recent years, Peng will now face Caroline Garcia, a winner in three sets against Julia Goerges.
RADWANSKA, PLISKOVA MADE TO WAIT TO SECURE FINALS QUALIFICATION
Radwanska and Karolina Pliskova are all but certain to participate in the end-of-season WTA Finals in Singapore, but they are not quite there yet after Madison Keys earned a straight-sets triumph over Kristina Mladenovic.
Had Keys lost on Monday, both Radwanska and Pliskova's places in the Finals would have been confirmed.
The latter duo are still on course to qualify this week, however. Should they reach the quarter-finals, Keys' results will be irrelevant.
Having dropped just four games in her first-round triumph over Wang Qiang, Radwanska again looked impressive as she beat Ekaterina Makarova 6-3 6-4.
Pliskova moved into round two with a 6-7 (7-9) 6-1 6-4 victory over Czech compatriot Lucie Safarova.
MUGURUZA SURVIVES SECOND-SET WOBBLE, KERBER BACK ON TRACK
Muguruza looked set to cruise through her second-round tie against Yulia Putintseva, but was twice broken when serving for the match in the second set.
The Spaniard eventually prevailed 6-2 7-6 (7-5) and said: "I'm very happy I could close the match in the tie-break because obviously it was getting tougher and tougher. I think she [Putintseva] was getting into the match a little bit more."
Muguruza's next opponent could be Petra Kvitova, who defeated world number one Angelique Kerber on her way to victory in last week's Wuhan Open.
Kerber bounced back from that loss on Monday, beating Katerina Siniakova 6-4 6-4. She faces Barbora Strycova in round two.
Daria Kasatkina, Daria Gavrilova, Timea Bacsinszky and Elina Svitolina also progressed to the second round, while Yaroslava Shvedova reached the last 16 with victory over Belinda Bencic.