LOS ANGELES (AFP) - Former world No. 1 Serena Williams suffered the most lopsided loss of her tennis career at the WTA San Jose event on Tuesday (July 31), losing the final 12 games in a shocking first-round upset by unseeded Johanna Konta.

The 23-time Grand Slam singles winner struggled mightily against the British No. 1 and former winner Konta, who crushed Williams 6-1, 6-0 in just 51 minutes on the main Stadium court.

Williams, who was playing her first match since dropping the Wimbledon final, is trying to regain her top form after missing most of the 2017 season due to a pregnancy.

The 36-year-old Williams tried to shrug off the worst loss of her Hall of Fame career.

"I don't know. I have so many things on my mind that I don't have time to be shocked about a loss where I clearly wasn't at my best," she said.

Williams, who was seeded sixth, managed just nine winners against 25 unforced errors and had her serve broken six times.

She looked nothing like the player who dominated the Tour for years, winning 72 singles titles and earning more than US$86 million (S$117 million) in prize money.

"She obviously wasn't playing her best level - nowhere near - so I tried to play the match on my terms and just do what I can out here," Konta said.

Both women were playing their first match since Wimbledon, where Williams finished runner-up to Angelique Kerber and Konta lost in the second round to eventual quarter-finalist Dominika Cibulkova.

Williams' loss comes just over a month after she complained that she is being discriminated against by the sport's drug testing programme because of the number of samples she is being asked to provide.

"Out of all the players it's been proven I'm the one getting tested the most. Discrimination? I think so," she tweeted.

She won this tournament the last three times she played it in 2011, 2012 and 2014 and won her only previous meeting with Konta 6-2, 6-3 in the 2017 Australian Open quarter-finals.

"I think she played better in this match than she did 18 months ago, so that's a good thing for her," said Williams.

Beat both sisters here

Konta won her maiden WTA title at this event in 2016 when it was held in Stanford University, defeating Williams' older sister Venus in the final.

She next faces Sofia Kenin, who earned an impressive win over qualifier Veronica Cepede Royg on Monday.

Konta played a superb match with 17 winners, nine unforced errors and won 83 per cent of her first-serve points.

This was one of the biggest wins of her career but the 27-year-old said she will not be celebrating too long because there is more work to do.

"I don't believe in big bang moments, or magic moments where the fingers click and it all comes together," she said.

"I believe in continuous hard work and every day perseverance, putting in the work and trusting the process.

"All this match has given me is the chance to play another one. I'm grateful for that opportunity and will work hard for another one."