Los Angeles - Defending champion Steve Johnson outlasted fellow American Taylor Fritz 7-5 6-7 (4/7), 6-2 on Saturday to reach the ATP US Clay Court Championships final.
The US sixth seed will play for his third ATP title and second in a row on Houston clay against American eighth seed Tennys Sandgren, who made a stunning run to the Australian Open quarter-finals in January.
Eighth-seeded Sandgren, ranked five spots behind Johnson at 56th, advanced to his first ATP final by downing Croatia's 39-year-old Ivo Karlovic 7-6 (7/2), 7-6 (7/4).
Johnson, whose ATP titles came in 2016 at Nottingham and Houston last year, broke Fritz to claim the first set in 50 minutes, but dropped four of the last five points in the tie-breaker to force a third set, which Johnson dominated to book his first match against Sandgren.
Fritz had beaten Johnson en route to his only ATP final in 2016 at Memphis.
Sandgren broke Karlovic to level at 4-4 in the opening set, denied the Croatian on a break chance in the ninth game and then dominated the tie-breaker, scoring five of the last six points.
Sandgren broke for a 2-1 edge in the second set but Karlovic broke back to level at 3-3, setting up the final tie-break. Sandgren took his lone point off Karlovic's serve for a 5-3 lead, then held twice on his next serves for the 97-minute triumph.
With a victory, 88th-ranked Karlovic would have become the oldest ATP finalist since Ken Rosewall won the 1977 Hong Kong title at age 43.
Karlovic settled for being the oldest ATP semi-finalist since Jimmy Connors in 1993 at San Francisco.