Heather Watson beaten in Maryland by Stephanie Dubois
Heather Watson's hopes of a third WTA quarter-final of the year ended with a three-set defeat by Canada's Stephanie Dubois at the Citi Open in Maryland.
The British number three looked on course for victory when she led by a set and a break, but with temperatures soaring she could not close it out.
Dubois saved nine of 10 break points in a tight second set before dominating the decider to win 3-6 7-5 6-1.
Watson is ranked 13 places higher than Dubois at 104 in the world.
The 19-year-old from Guernsey had won their only previous meeting 6-1 6-0 earlier this year and took five straight games on her way to securing the opening set.
But Dubois, 24, held on gamely in a tense second before reeling off six games in a row from 5-5 as she went on to win in a gruelling two hours and 40 minutes, which included a break before the decider under the extreme heat rule.
Dubois said after her win: "I never gave up. She was playing well. I stayed aggressive and I managed to play more aggressively in the second set. She is consistent so I knew I had to make something."
Watson was the last remaining Briton in the draw after Elena Baltacha and Anne Keothavong lost in earlier rounds.
Former world number one Serena Williams advanced to the quarter-finals of the Bank of The West Classic in California by beating Russia's Maria Kirilenko 6-2 3-6 6-2.
It sets up a last-eight clash for Williams with another former world number one, Maria Sharapova.
Williams has won six of her eight matches against Sharapova and is aiming to win her first WTA Tour title since last year's Wimbledon.
"I'm sure we'll both go out and do the best each of us can. It's nothing personal. It's my job and I want to get paid. I leave it on the court.
"If I say I just want to play well, it's a lie. Would I be happy about losing? When I'm playing I get a little psycho and on the edge."
Williams is currently ranked 169th in the world after taking time off the tour to recover from foot surgery and a blood clot on her lung.
Top seed Victoria Azarenka lost to New Zealand qualifier Marina Erakovic after taking the first set. Erakovic, ranked 121st in the world, lost the first set 6-4, but recovered to win the next two sets 7-5 6-2.
Fourth seed Samantha Stosur of Australia also lost, 6-3 7-5 to Germany's Wimbledon semi-finalist Sabine Lisicki.