John Isner failed to advance to the Memphis Open semi-finals but Kei Nishikori powered into the final four in Buenos Aires.
American second seed Isner went down in three sets to countryman Donald Young on Friday.
Despite not giving up a break point, 2010 runner-up Isner - who fired down 19 aces - still lost 7-6 (7-5) 3-6 7-6 (8-6) in the quarter-finals of the ATP 250 event.
After upstaging Isner for the first time in his career, Young will next face Ryan Harrison, who beat Damir Dzumhur 6-3 6-4.
Mikhail Kukushkin and Nikoloz Basilashvili will contest the other semi-final.
Kukushkin dropped just 10 points as he eased past fourth seed Steve Johnson 6-0 6-4.
After stunning top seed Ivo Karlovic, Basilashvili saw off qualifier Matthew Ebden 7-6 (7-3) 6-4.
At the Argentina Open, Nishikori powered into the semis at the expense of fellow seed Joao Sousa.
Top seed Nishikori accounted for Sousa 6-1 6-4 as he seeks his 12th ATP title.
Next up for Nishikori is Carlos Berlocq, who topped Thiago Monteiro 3-6 6-2 6-3.
Pablo Carreno Busta - the fourth seed - overcame fifth seed Albert Ramos-Vinolas 6-3 4-6 6-4 in their all-Spanish affair, while Alexandr Dolgopolov defeated Gerald Melzer 7-5 6-4.