Sebastian Korda and Brandon Nakashima set up an all-American semifinal at the Next Gen ATP Finals by winning their last round-robin matches on Thursday.
The 21-year-old Korda, the son of 1998 Australian Open champion Petr Korda, methodically picked apart local favourite Lorenzo Musetti of Italy in a 4-2, 4-3 (4), 4-2 victory to win Group B with a record of 3-0.
Earlier, the 20-year-old Nakashima defeated Holger Rune 3-4 (3), 4-1, 4-1, 4-3 (1) to secure second place in Group A behind top-seeded Carlos Alcaraz at the season-ending tournament for the top 21-and-under players.
Korda won his first career title on Italian soil in Parma in May, after which Nakashima reached back-to-back championship matches at Los Cabos, Mexico, and Atlanta to become the youngest American to play in multiple finals since Andy Roddick in 2001-02.
Also reaching the last four was Sebastian Baez of Argentina, who beat Hugo Gaston 4-3(2), 4-2, 4-2 to finish second in Group B behind Korda and set up a semifinal with Alcaraz.
Alcaraz, who had already won Group A, produced 22 forehand winners in a 4-0, 4-1, 2-4, 4-3 (3) win over Juan Manuel Cerundolo, who was already eliminated.
The victory made the 18-year-old Alcaraz the youngest player to earn 30 tour-level wins in a season since Rafael Nadal went 30-17 in 2004 when he was 18. Alcaraz is also 30-17 entering the semifinals.
There are different rules at the Next Gen tournament, including on-court coaching, no-Ad scoring, medical timeout limits, and Hawk-Eye making all the line calls.
The most drastic change is the shorter first-to-four set, with a tiebreaker at 3-3.