Konta, 28, was beaten 6-3 3-6 7-5 by Swedish qualifier Rebecca Petersen.

The world number 14 trailed 4-1 in the final set but recovered to level at 4-4 before Petersen took her fourth break point of the match to seal victory.

Konta lost to Ukrainian teenager Dayana Yastremska in the first round of the Rogers Cup in Toronto last week.

Seven-time Grand Slam champion Venus Williams, 39, ended a four-match losing streak by beating fellow American Lauren Davis 7-5 6-2.

Maria Sharapova overcame fellow wild card Alison Riske 6-3 7-6 (7-4) to set up a second-round tie with world number two Ashleigh Barty, who defeated the Russian at this year's Australian Open.

Former world number one and five-time Grand Slam champion Sharapova, 32, succeeded on her fourth match point to set up the meeting with top seed Barty.

Australian Barty, 23, defeated Sharapova in the fourth round in Melbourne in January.

Another former world number one, Caroline Wozniacki, lost to Ukrainian teenager Dayana Yastremska 6-4 6-4 in the final action of the day.