Johanna Konta's challenging start to the hard-court season continued on Monday after she was knocked out in the first round of the Cincinnati Masters by Swedish qualifier Rebecca Petersen.
The British No 1 was seeded 14th for the tournament but found herself on the wrong end of the outing, losing 6-3 3-6 7-5.
Konta was on the ropes early after losing the first set. However, at 2-2 in the second, she reeled off three successive games, before closing it out 6-3.
The Brit broke Petersen again in the first game of the third but subsequently surrendered four games on the trot. In the topsy-turvy clash, Konta fought back to five games apiece before Peterson found the extra gear needed to secure victory.
The British No 1's loss, in which she produced 45 unforced errors, follows a first-round loss at the Rogers Cup against Dayana Yastremska last week.
Meanwhile, Maria Sharapova, who was playing in the tournament for the first time in five years, prevailed over fellow wildcard Alison Riske 6-4 7-6 (7-4)
The 2011 Cincinnati champion showed her fortitude during the second set as she twice came back from a break down, missed out on a match point chance at 6-5, and it took until her third match point in the tie-break to clinch the win.
The result means that she will face the top-seed Ashleigh Barty which should be an intriguing clash after the pair met in the fourth round of the Australian Open earlier in the year.
"She's one of the few players that have really dominated and has been impressive to see," said Sharapova of her next opponent, Barty.
"I'm the one coming in with not a lot of match play, but I'll have to figure it out. If not, it's going to be too late."
Elsewhere in the draw, Yastremska took just over 90 minutes to send the former world No 1 Caroline Wozniacki packing 6-4 6-4, while Venus Williams was also triumphant.
Williams prevailed over the American qualifier Lauren Davis 7-5 6-2 and will take to Centre Court against Kiki Bertens directly after her sister on Tuesday.
Bertens is the defending champion in Cincinnati after a stunning run last year that included a final victory over Simona Halep.
Serena Williams opens her account against Zarina Diyas and will be eager to put behind her the painful memories of the back injury that she suffered in the final of the Rogers Cup.