Ben Shelton, the No 6 seed, reached the last eight of the Swiss Indoors by winning against Swiss wildcard Stan Wawrinka 7-6 (2), 7-5.
It was a very close contest, with the young American edging victory in two tight sets to deny Wawrinka a second famous win in front of his home fans in Basel.
The Swiss veteran had ousted Mannarino in the previous round, becoming the oldest ever winner as the Swiss Indoors Open. But, despite a valiant performance of high quality, youth ultimately prevailed as Shelton held firm in the decisive moments to clinch an impressive victory.
The world No 23 progresses to the quarter-finals where he will face Russian Andrey Rublev, the top seed, next.
In the previous round of the Basel tournament, the 22-year-old American defeated Argentinian Tomas Martin Etcheverry (6-3, 6-4).
Wawrinka, ranked No 169, edged out Frenchman Adrian Mannarino (6-3, 3-6, 7-5) in the previous round of the Basel tournament.
Basel ATP 500, other second-round results (St. Jakobshalle., hard, EUR 2.385.100, most recent results first):
- Felix Auger-Aliassime vs. Giovanni Mpetshi Perricard
- Dominic Stricker vs. Holger Rune
- Denis Shapovalov vs. Roberto Bautista Agut: thursday
- Arthur Fils (7) beat Pedro Martinez Portero: 7-5, 6-3
- Stefanos Tsitsipas (3) beat Botic van de Zandschulp (Q): 7-6 (3), 7-5
- Andrey Rublev beat Alejandro Tabilo: 7-6 (3), 6-1
- David Goffin (LL) beat Ugo Humbert (5): 3-6, 6-1, 7-6 (7)