World number one Ashleigh Barty was beaten for the first time in two months on Monday as Alison Riske claimed a superb fourth-round victory at Wimbledon.
The unseeded American, who had only reached this stage of a grand slam once before in her career, recovered to win 3-6 6-2 6-3 in 97 gruelling minutes on No.2 Court.
Having already beaten seeds Belinda Bencic and Donna Vekic at these championships, Riske will face Serena Williams, who breezed into the last-eight with a 6-2 6-2 victory over Carla Suarez Navarro, in her first major quarter-final.
Barty, who had not even dropped a set since the semi-finals of the French Open last month before this meeting, began with four consecutive aces as she battled to a 4-1 lead, before closing out the opening set as Riske missed a backhand return at the end of another love service game.
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The Australian's backhand slice had been causing Riske some problems but she was off balance when her opponent responded with a string of emphatic ground strokes to move 4-1 ahead in the second set.
Riske then levelled the match with a simple passing shot after Barty, who won only two points at the net in set two, failed to do enough with a forehand volley.
A hard-fought third set swung Riske's way when she broke for a 5-3 lead with a fine forehand down the line, and last month's Libema Open champion claimed victory on her first match point as Barty sent a forehand into the tramlines.
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"I've just been ready to battle every day I go out there and that's really shown in all my matches. I couldn't be more proud of myself," said Riske.
"I couldn't begin to say what it means to me. To overcome the matches the way that I have ultimately is what I'm most excited about. Being in the last eight at Wimbledon isn't too bad!"
Later in the day, previous form offered little indication that Williams' pursuit of a record-equalling 24th grand slam singles title would be knocked off course by Navarro, who had failed to win a set in their six previous meetings.
This proved a similarly forlorn assignment for the 30th seed, despite some good moments in the middle of each set as errors flowed from her esteemed opponent's racquet more frequently than might have been expected.
Williams will face Alison Riske in the quarter-finals after her compatriot downed world number one Ashleigh Barty via a gruelling 96-minute encounter on Number Two Court.
Over on Centre, early breaks at the start of each set were key - Suarez Navarro unhelpfully setting the tone for herself with a double fault on the opening point.
Williams converted her second break point but her Spanish opponent avoided any further damage until the seventh game.
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A crunching backhand gave Williams the opening after a third deuce and Suarez Navarro sent her own shot from that wing into the net.
The contest threatened to race away from the 30-year-old as she coughed up two more service games to be down 0-3 in the second.
Things had arguably become too easy for Williams, who was jolted by being broken to love in the next game and needed an assured passing shot to end a run of six consecutive points lost.
Serena reacted exuberantly to a driven forehand winner that saw off another service wobble to set up a hold at 4-2 and a glorious cross-court backhand to break for a fifth time in the contest meant Suarez Navarro's race was run.
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Serena reacted exuberantly to a driven forehand winner that saw off another service wobble to set up a hold at 4-2 and a glorious cross-court backhand to break for a fifth time in the contest meant Suarez Navarro's race was run.
