Defending champion Kei Nishikori will face big-serving Croatian Ivo Karlovic in Sunday’s final of the ATP US National Indoor Championships.

Japanese star Nishikori, top seeded for the first time at an ATP event, defeated American Michael Russell 6-3 6-2 in his semi-final on Saturday while Karlovic advanced when Taiwanese fourth seed Lu Yen-Hsun retired after losing the first set 6-1.

World No.16 Nishikori is seeking his fourth ATP crown while Karlovic, ranked 80th, is after his sixth career title.

Karlovic, whose serve has not been broken this week, has won both prior meetings against Nishikori, taking a 2012 Davis Cup match and a Delray Beach triumph last year when the Asian star retired down 5-4.

Nishikori broke Russell late in the first set and to open the second. The 24-year-old dropped only three points on his serve in the second set to win his first meeting with 98th-ranked Russell in only 64 minutes.

Karlovic blasted seven aces past 54th-ranked Lu and lost only one of 17 points on his own serve to win the only set in 29 minutes.

Lu’s serve was weak from the start and he ended a rally by making a video appeal on a line call that failed, handing Karlovic a break for a 3-1 lead.

Lu, 30, had beaten Karlovic, 34, in their only previous meeting, taking the last two of three tiebreakers to win at Queen’s on grass in 2012.

Nishikori won two singles matches and shared a doubles triumph to spark Japan over Canada in Davis Cup play earlier this month.

His ATP titles came as a teen at Delray Beach in 2008, Tokyo in 2012 and last year at Memphis where he dispatched Spain’s Feliciano Lopez in the final.

Karlovic snapped a five-year ATP title drought last year at Bogota. He also won crowns at Nottingham in 2007 and 2008 and Stockholm and Houston in 2007.