Petra Kvitova powered past Wimbledon champion Garbine Muguruza in Sunday’s US Open night session to reach the quarter-finals.
The Czech 13th seed fought back from 1-4 down in the first set to win 7-6 (7-3) 6-3.
It is Kvitova’s best effort at a Grand Slam since she reached the last eight in New York two years ago.
The 27-year-old returned to action in May following a knife attack at her home last December.
She required extensive surgery on her left hand and confirmed only last week that it still did not feel completely back to normal.
However, the evidence of Monday night is that Kvitova is once again capable of the irresistible attacking tennis which brought her Wimbledon titles in 2011 and 2014.
“Sometimes I just really feel that the touch is there – the strength, the aggressive kind of game plan,” said Kvitova.
“It’s there, which I really, really appreciate. It took me a while to find it. Luckily I found it in a Grand Slam, which is nice.”
She goes on to face Venus Williams in the last eight on Tuesday.
“It will be a great match for me to step on Arthur Ashe Stadium again – probably – and play there,” said Kvitova.
“Venus has something special which the other players doesn’t have.”
The defeat is a blow to Muguruza’s hopes of becoming world number one, although she remains in pole position as things stand.
Czech Karolina Pliskova must reach the final to hang on to the number one spot, and Ukraine’s fourth seed Elina Svitolina needs to reach the semi-finals to stay in contention.
Both women play their fourth-round matches on Monday.
“We’ll see – I’ll keep an eye now on the scores,” said Muguruza.
Muguruza, 23, was widely viewed as the title favourite and next number one in waiting, until she ran into the blistering shot-making of Kvitova.
Both women finished with 18 more errors than winners, but Kvitova’s attacking intent was rewarded with 24 winners to seven.
Muguruza eased into an early 4-1 lead and had three break points for 5-1, but Kvitova held firm and clenched her fist after a forehand winner kept her in touch.
Kvitova would get the chance to serve for the set six games later but played a poor game, and fell 2-0 behind in the tie-break.
From that moment on, the Czech was in command, moving 5-3 clear after Muguruza steered a volley long and converting her first set point with a smash.
An early break for the Spaniard in the second set was quickly handed back, and Muguruza’s forehand would begin to falter – she ended with 16 errors on that side.
Four straight games put Kvitova on the brink of victory and she got across the line in dramatic circumstances, saving three break points before Muguruza finally succumbed.
“Even though I was winning 4-1, I think she was playing great,” said Muguruza.
“I felt like she was superior today. You know, great for her. She played very good.”