Belinda Bencic continued the form that saw her beat four top-10 players on her way to winning the Dubai Duty Free Tennis Championships in February, as she shocked world No 1 Naomi Osaka in the fourth round at Indian Wells.
Osaka never found her rhythm against the Swiss, who used her precise groundstrokes and pinpoint serving to topple the defending champion 6-3, 6-1 in just over an hour.
The result continues Osaka's tough start to life at the top of the WTA rankings, having been beaten in straight sets by Kristina Mladenovic in the second round in Dubai.
"She sort of came out there really aggressive," Osaka said. "She sort of knew what she wanted to do more than me."
Osaka fired her coach and lost in the first round in Dubai before taking on a new coach, Jermaine Jenkins, who came on court during the match.
"It's always a bit tricky whenever you change someone in the team, but I feel like for a first tournament we did really well," Osaka said. "We communicate well, so that's a really big plus. I don't think it's going to be difficult."
Bencic, 22, who has risen as high as world No 7 before struggling with injuries, hit a forehand volley on match point to seal the win and send her into the quarter-finals.
"I was really happy with the performance," Bencic said. "It wasn't as easy as the result is saying and I didn't expect it like that, so definitely happy."
The world No 23 will next go up against world No 5 Karolina Pliskova for a place in the semi-finals.
Osaka was not the only top women's player shocked on Tuesday in California as world No 2 Simona Halep, the 2015 winner, fell to Czech teenager Marketa Vondrousova 6-2, 3-6, 6-2.
Halep, who received treatment for blisters on her foot during the match, hit just six winners and piled up 36 unforced errors.
"I was running too much and too defensive," she said. "But it's everything I could do today."
The win was 19-year-old Vondrousova's first over a top-10 player in six attempts.
"I'm playing well all week but this is something special because she's number two in the world," Vondrousova said. "I was just fighting for every point."
