Kerber needed to produce her best tennis.
In the quarterfinals, Keys will play the victor of the later fourth-round match between 2016 Australian champion Angelique Kerber and Hsieh Su-wei.
Sandgren's next opponent is to be world No. 58 Hyeon Chung of South Korea, who defeated Serbian star Novak Djokovic in straight sets.
Djokovic was playing his first competitive tennis since Wimbledon last July, and had to remodel his service swing to take some load off his injured right elbow. After sealing the match in style with a thumping forehand victor from the baseline, Sandgren threw down his racket and raised his arms aloft in joy. Goodness gracious! I knew I had to take my chances and he, from behind the court, would outlast me. "Dreams come true tonight". "Impressive. Whenever he was in trouble, he came up with some unbelievable shots".
"I was telling myself to stay calm, don't burn too much energy on stuff you don't need to, just focus on what you have to do".
Sandgren outlasted No. 5 seed Dominic Thiem in a five-set thriller 6-2, 4-6, 7-6 (7-4), 6-7 (7-9), 6-3 to advance to the quarterfinals.
"I didn't think I'd see you guys again", quipped Sandgren as he walked into his press conference.
Ironically, he played no small part in 26-year-old Sandgren's blossoming in Melbourne. The only American man, Tennys Sandgren, plays Dominic Thiem later Monday.
The decider went on serve until the sixth game when Sandgren got the decisive break to lead 4-2 and went on to serve out the set and match winning it 6-3 and a spot in the quarter-finals.
The 19-time major victor had never played Fucsovics but had beaten his coach - Attila Savolt - here in the second round in 2002.
Tomas Berdych is back in the quarterfinals of the Australian Open for the seventh time after a 6-1, 6-4, 6-4 win over Fabio Fognini.
Angelique Kerber, the only Grand Slam singles victor remaining in the women's draw, was up earlier, and got a serious wakeup call.
For a while it appeared former the Australian and U.S. Open champion's tournament could unravel against No. 88 Hsieh Su-wei, a former top-ranked doubles player with a double-handed grip on both sides.
Halep, who could lose her No. 1 ranking if she doesn't win the title here, extended her record of not having lost a fourth-round match on hard courts at a Grand Slam since the 2013 U.S. Open - she's 10-2 in round-of-16 matches. "I was feeling I was running everywhere".
"This is my first main draw at the Australian Open and now I'm in the quarter-finals".
Hsieh, contesting the fourth round in a major for the first time in a decade, certainly made the most of her time back in the spotlight.
"I like to play freestyle", Hsieh, a two-time Grand Slam doubles titlist, explaining her unusual array of shots. "If I wake up now, I'm going to be real upset".
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