Serena Williams faced her most lopsided defeat of her career, losing the opening match of the Mubadala Silicon Valley Classic on Tuesday to Johanna Konta, 6-1, 6-0.
The match ended in 52 minutes as Williams played her forehand into the net.
The sixth-seeded Williams is a three-time champion in the Bay Area.
This marked the fifth tournament for the 36-year-old Williams since giving birth to her daughter, Alexis Olympia, last September. It's her first tournament since her straight-set Wimbledon loss to Angelique Kerber.
After a routine hold to start the match, Williams lost the next 12 games in a one-sided affair that lasted less than an hour, stunning the partisan crowd of 3,000 at San Jose State University.
Those expecting to see Williams continue to battle back after a difficult child birth last year were left disappointed as the 36-year-old committed 25 unforced errors to Konta's nine and managed to put just 41 percent of her first serves into play.
"I think she played well in the second set and I think I wasn't sharp at all in the first set and she got confident and she clearly ran away with it," the American said after the loss.
With the loss Williams falls to 11-4 since returning to the tour and will look to bounce back when she competes in the Cincinnati Open next month, another hard court tune-up ahead of this year's U.S. Open.
Konta will face American teenager Sofia Kenin in the second round.
Donald upsets Wawrinka in Washington
Three-time Grand Slam champion Stan Wawrinka suffered a first-round exit at the Citi Open in Washington on Tuesday, going down 6-4, 6-7(5), 7-6(3) to qualifier Donald Young.
Wawrinka, who was knocked out in the second round of Wimbledon earlier this month, is still trying to find fitness and form after undergoing two operations on his knee last year.
The 29-year-old Young went through the match without losing serve and will next face Japan's Kei Nishikori.
Wawrinka did not see his first break point opportunities of the match against the left-hander until 3-3 in the third set but squandered both with unforced errors.
Young had looked on course to take the match in straight sets but lost the last three points of a second-set tiebreak. He did not make the same mistake in the third set tiebreaker, winning the last four points to advance.
"I've been losing so much, why not just buckle down and fight and compete," said Young, who had started the year 1-9 and has fallen to No. 234 in the world.
"I figured that if I can get into that position once, I can do it again.
"It's rough. There are times you're home and you don't want to go to the next tournament, you don't want to practice, you don't want to work out," he added.
"But I have great people around me and they keep pushing me."
Donald will look for his first career win against 28-year-old Nishikori, who has won all six of their previous meetings including a marathon three-setter at the Citi Open last year.
Stephens gets hardcourt campaign off to winning start
Sloane Stephens began the build-up to her US Open title defence with a 7-5, 6-4 win over wildcard Bethanie Mattek-Sands back on the American hardcourts at the Citi Open in Washington, DC on Tuesday.
Stephens, who won her first career title in Washington in 2015, finished off Sands with her sixth ace of the match to book a second round meeting with Germany's Andrea Petkovic, who was a 6-1 6-1 winner over American Jamie Loeb.
It marked the fifth time in six meetings that Stephens has beaten fellow American Sands, all of the five other matches having been played on clay.
The third ranked Stephens is the top remaining seed in the draw after world number two Caroline Wozniacki withdrew earlier on Tuesday with a right leg injury.
Elsewhere third-seeded Naomi Osaka won her debut match at the Citi Open with a 6-2, 7-6(4) over American Bernarda Pera while eigth-seeded Donna Vekic broke Caroline Dolehide three times en route to a 6-3, 6-4 victory.
Romanian Ana Bogdan stunned fourth seed Ekaterina Makarova 7-6(2), 6-3 and Kazakh Yulia Putintseva outlasted German Tatjana Maria 6-3, 3-6, 6-4
Murray adds China Open to schedule
Former world number one Andy Murray has added the China Open to his schedule in October as he continues his return from hip surgery earlier this year.
Murray, who played three matches in late June but withdrew from Wimbledon as it came too early in his recovery process, made his return to hardcourt tennis on Monday at the Washington Open where he battled to a first-round victory over Mackenzie McDonald.
"The China Open ... has announced the entry of three-time Grand Slam champion and former world number one Andy Murray to the 2018 player field," tournament organisers said on their website.
Murray, who will face fellow Briton Kyle Edmund in the second round of the Washington Open, is expected to play in Toronto and Cincinnati before the final Grand Slam of the year at the U.S. Open.
The China Open, which will be held from October 1 to 7, will also feature current world number one and defending champion Rafa Nadal and Argentine Juan Martin Del Potro.