An out-of-sorts Alexander Zverev was dumped out of the US Open third round on Saturday by compatriot Philipp Kohlschreiber in four sets while, in the women's circuit, fourth seed Angelique Kerber's bid for consecutive Grand Slam titles ended when 29th seed Dominika Cibulkova defeated the German 3-6, 6-3, 6-3.
Zverev and Kohlschreiber were meeting for the third time with the former having won the last two but there was little the fourth seed could do to stop Kohlschreiber, who won 6-7 (1), 6-4, 6-1, 6-3 at Louis Armstrong Stadium.
Zverev showed once again that he is not yet the finished article. He teamed up with Ivan Lendl ahead of the US Open, bringing the eight-time Grand Slam champion on to his coaching staff in an attempt to take the next step in his career.
"It's a process," Zverev told reporters. "I took [Lendl] to do well at slams, but this is a process. You can't expect to kind of immediately see the results.
"He told me before the tournament started that, hopefully you'll do very well at the US Open, but we're more looking towards... being at the latter stages, competing for slams next year."
In the Kerber-Cibulkova game, momentum shifted in Cibulkova's favour in the third set when she broke to take a 4-2 lead and held for 5-2.
The Slovakian fell to the ground and let out a scream when Kerber sent a backhand wide to end a quality match between two of the game's premier counter-punchers.
Earlier, defending champion Rafael Nadal became one of the first into the quarterfinals after seeing off Nikoloz Basilashvili 6-3, 6-3, 6-7 (6/8), 6-4.
He will face Dominic Thiem, one of only three men to defeat him in 2018. Nadal defeated Thiem in the Roland Garros final in June when he captured an 11th French Open.
Thiem booked his place in the quarter-finals for the first time with a 7-5, 6-2, 7-6 (7/2) win over 2017 runner-up Kevin Anderson.
Five-time champion Roger Federer found a new way to amaze as he reached the US Open last 16 with a 6-4, 6-1, 7-5 victory over Nick Kyrgios.
The second-seed withstood an early onslaught from the Aussie, firing 51 winners that included an unlikely forehand flicked around the net post that Kyrgios couldn't believe.
"Definitely a bit of luck and good feet," Federer said of the shot that had Kyrgios wide-eyed.
The wonder shot was the talk of the day -- but the night belonged to 2014 champion Marin Cilic and Alex de Minaur, who battled for four hours on Louis Armstrong Stadium before Cilic pulled off a 4-6, 3-6, 6-3, 6-4, 7-5 win.