Japanese teenager Kei Nishikori shocked the heavily-favoured James Blake in the final of the Delray Beach International Championships to post the biggest victory of his young career on Sunday.

The 18-year-old qualifier rallied from a set down to stun the top-seeded American 3-6 6-1 6-4 in the hardcourt event for his first career title.

Nishikori became the first player from Japan to win an ATP Tour event since Shuzo Matsuoka at Queen's in June 1992, which was also the last time a Japanese player even reached a final.

Nishikori advanced to Sunday's final with a victory over number three seed Sam Querrey, also from the United States, in the semi-finals on Saturday. He saved four match-points in the 4-6 6-2 7-6 (9/7) victory.

Blake, who lost in the final here for the second straight year, reached the title match without facing a seeded player. Last year, he fell in the final to Belgium's Xavier Malisse.

Ranked 12th in the world, Blake was seeking his 11th career title and third on home soil.

He was making his first appearance since helping the United States open the defence of their Davis Cup title with a 4-1 first-round win over Austria last week.

Nishikori admitted he never expected to defeat Blake, having struggled on the Challenger circuit.

"Last night, I tried to imagine winning but I couldn't do it," he said. "I was so nervous in the first set. During the second set, I started to feel more confident."

At 18 years, one month and 18 days, Nishikori becomes the youngest winner of an ATP tournament since Lleyton Hewitt (16 years, 10 months) in Adelaide in 1998. He is projected to climb from a world ranking of 244th to 122nd.

"This is the best tournament in my life," he added. "I am so happy right now. I played eight matches in nine days, but I didn't feel tired."

Nishikori advanced to Sunday's final with an upset of Sam Querrey, also from the United States, in the semi-finals on Saturday. He saved four match points in the 4-6, 6-2, 7-6 (9-7) victory.

Nishikori showed similar composure against Blake, saving three break points in the second game of the final set to get even. He then broke Blake and held to serve to take a 3-1 lead. Nishikori served out the match at love.

"Anytime you save four match points in the semis, you are playing with house money," Blake said of his opponent. "He hit cold winners at times you wouldn't expect them."

Blake reached the title match without facing a seeded player.

Last year, he fell in the final to Belgium's Xavier Malisse.

Blake managed only one break against Nishikori and converted only 44% of his second serves.

"One break in a three-set match says a lot on my part," Blake said. "I felt he'd play care-free and play like he had nothing to lose."

Nishikori admitted he never expected to defeat Blake, having struggled on the Challenger circuit.

"Last night, I tried to imagine winning but I couldn't do it," he said. "I was so nervous in the first set. During the second set, I started to feel more confident."

At 18 years, one month and 18 days, Nishikori becomes the youngest winner of an ATP tournament since Lleyton Hewitt (16 years, 10 months) in Adelaide in 1998. He is projected to climb from a world ranking of 244th to 122nd.

"This is the best tournament in my life," he added. "I am so happy right now. I played eight matches in nine days, but I didn't feel tired."

Nishikori advanced to Sunday's final with an upset of Sam Querrey, also from the United States, in the semi-finals on Saturday. He saved four match points in the 4-6, 6-2, 7-6 (9-7) victory.

Nishikori showed similar composure against Blake, saving three break points in the second game of the final set to get even. He then broke Blake and held to serve to take a 3-1 lead. Nishikori served out the match at love.

"Anytime you save four match points in the semis, you are playing with house money," Blake said of his opponent. "He hit cold winners at times you wouldn't expect them."

Blake reached the title match without facing a seeded player.

Last year, he fell in the final to Belgium's Xavier Malisse.

Blake managed only one break against Nishikori and converted only 44% of his second serves.

"One break in a three-set match says a lot on my part," Blake said. "I felt he'd play care-free and play like he had nothing to lose."