America's Donald Young stunned South African fourth seed Kevin Anderson 6-2, 7-6 (8-6) to reach the Basel Indoors quarter-finals on Thursday.
Young, the world number 49, goes on to face 29th-ranked compatriot Jack Sock who withstood 14 aces from fellow American and sixth seed John Isner's racquet in a 7-6 (7-5), 6-3 win.
Sock earned his second win from six attempts against Isner as the 23-year-old claimed his 34th match of the season.
Seventh-seeded 2014 US Open winner Marin Cilic earned a place in the quarter-finals, beating Russian Teimuraz Gabashvili 6-3, 6-1 but managing just seven aces in the process.
Cilic goes on to face 14-time major winner Rafael Nadal on Friday for a place in the semi-finals.
Serve king Ivo Karlovic enjoyed a 7-6 (7-4), 7-5 win over Serbia's Dusan Lajovic, with the Croatian firing 25 aces just a day after knocking out Swiss second seed Stan Wawrinka with 21.
Anderson, Isner and Karlovic have each delivered more than 1,000 aces in ATP play this season, the first time that mark has been achieved since records began in 1991.
Later Thursday, fifth-seeded Richard Gasquet was trying to extend his outside bid for a place in the season-ending World Tour Finals.
The Frenchman needs to win the Basel title and next week's Paris Masters to have a chance to claim a place in the eight-man London showpiece.
With the last two spots in the season wrap-up still in play, the situation was complicated by the pullout in Valencia of David Ferrer.
The 33-year-old Spaniard, who won the Vienna title last weekend, was suffering with an elbow injury, a problem which forced him off court for three months this season.
Ferrer stands joint seventh in the points chase, level with Japan's Kei Nishikori who is also out injured.
Gasquet was looking for a win over Austrian Dominic Thiem to keep his year-end hopes alive.
Hometown hero and six-time winner Roger Federer was also in action later Thursday against German Philipp Kohlschreiber, 10 times a loser to the top-seeded Swiss.