Amanda Anisimova claimed the scalp of an out-of-sorts Elina Svitolina and Petra Martic was among the other seeds who crashed out of the Qatar Open on Monday.
Svitolina has endured a disappointing start to the season and there was no upturn in fortunes in Doha, where she was beaten 6-3 6-3 in the second round.
Martic got past Barbora Strycova in the first round in Dubai last week, but the Czech gained revenge with a 4-6 6-1 6-1 victory to move into round two.
Alison Riske, the 13th seed, lost an all-American encounter with Jennifer Brady, while 17th seed Donna Vekic went down in straight sets to Iga Swiatek.
Kiki Bertens had no such trouble, winning 6-2 6-4 against Karolina Muchova to reach the third round, while Elena Rybakina came from a set down to oust Sorana Cirstea.
TEENAGER ANISIMOVA STARTLES SVITOLINA
Svitolina has won just four matches this year and it was 18-year-old Anisimova who inflicted another blow on the world number seven.
The American teenager claimed a first top 10 win of 2020, taking just 64 minutes to sail through in her first appearance in this tournament.
She struck 27 winners and saved all three break points faced and will take on either Swiatek or Svetlana Kuznetsova for a place in the quarter-finals.
RYBAKINA ROARS BACK
Rybakina suffered the agony of a defeat to Simona Halep in the final of the Dubai Tennis Championships on Saturday.
The 20-year-old rising star from Kazakhstan, up to a career-high 17th in the rankings, gave another demonstration of her fighting spirit by roaring back to beat Cirstea 3-6 6-3 6-1.
Rybakina already has 20 victories to her name this season and will be expected to make it 21 when she takes on Alison Van Uytvanck in the second round.
SUAREZ NAVARRO SETS UP BATTLE OF FORMER CHAMPIONS
Carla Suarez Navarro coasted to a 7-5 6-0 first-round victory over Zhang Shuai on day two of the tournament.
The Spaniard's reward will be a tough encounter with Petra Kvitova, two players who have experienced glory in Doha.
Suarez Navarro won the tournament in four years ago and Kvitova was crowned champion in 2018.