Unseeded Australian Daria Gavrilova caused the first upset of the WTA Western and Southern Open after ousting Kristina Mladenovic, while Petra Kvitova rallied.
Gavrilova powered into the second round with an upset win over 13th seed Mladenovic in Cincinnati on Monday.
Two-time Wimbledon champion Kvitova came from behind to trump Anett Kontaveit in three sets.
There was no revenge for Coco Vandeweghe, who went down to Madison Keys at the Premier event.
MLADENOVIC STUNNED BY GAVRILOVA
Gavrilova was too good on Monday, weathering a late comeback to triumph 6-0 7-6 (8-6).
The world number 26 broke three times in a first-set onslaught and she held on to condemn Mladenovic to consecutive opening-round exits, having lost in Toronto.
Next up for Gavrilova is Camila Giorgi, who accounted for Bastad champion Katerina Siniakova 6-2 6-2.
TWO TIGHT SETS AND A BAGEL
There was a surprising conclusion to Lesia Tsurenko's match against Anastasia Pavlyuchenkova.
The pair shared the opening two sets - closely-contested affairs with the Russian taking the first 7-5 having saved one set point with Tsurenko serving at 5-4 40-30.
The Ukrainian fought back to level the match on a tie-break - having again failed to serve out the set earlier - and an epic decider seemed on the cards.
But that proved to be far from the case as Tsurenko bagelled her opponent in the last - saving six break points in game three - to go through courtesy of an odd-looking 5-7 7-6 (8-6) 6-0 scoreline.
KVITOVA'S COMEBACK CRUSHES KONTAVEIT
Tsurenko was not the only woman to fight back from a set down to progress, however, as Petra Kvitova recovered from a poor first set to oust Anett Kontaveit 1-6 7-6 (7-2) 6-3.
It looked as though Kvitova - still feeling her way back into the Tour after the shocking knife attack in December which threatened to end her career - would be making an early exit as Kontaveit took a 4-2 lead in the second.
But the Czech displayed her trademark battling qualities to break back swiftly, before dominating the tie-break and then the decider to book her place in the second round.
KEYS STOPS VANDEWEGHE... AGAIN!
Vandeweghe just cannot get past fellow American Keys.
For the second time in as many weeks, 16th seed Keys defeated Vandeweghe, this time winning 6-4 3-6 6-3.
Keys trumped Vandeweghe to claim the Bank of the West Classic title less than two weeks ago in Stanford.