The 18-year-old, who began the season by startling James Blake, became the first man from Japan in 35 years to reach the third round at the US Open with a win on Thursday.
Nishikori led Roko Karanusic 6-1, 7-5 when his Croatian opponent pulled out with leg trouble.
"I wasn't playing well on Friday. But he retired, and I was just lucky," Nishikori said.
Next up for the world's 126th-ranked player is a sterner test _ fourth-seeded David Ferrer. No man from Japan has reached the fourth round at this event since the Open era began in 1968. Jun Kamiwazumi was the last Japanese man to make it to the third round, back in 1973. Japanese women have fared better, with Ai Sugiyama seeded 30th at this Open.
Nishikori started the year with quite an upset, qualifying at Delray Beach, Florida and then beating the top-seeded Blake for the title. He became the youngest player to win an ATP title since Lleyton Hewitt did it at 16 in 1998.
More than a dozen Japanese fans chanted "Nippon! Nippon!" from the seats as Nishikori won that event in February. Nishikori's parents were at Flushing Meadows to watch his latest victory. Now based in the United States, he plans to take a vacation back in Japan later this year.
"Sometimes I get homesick," he said.
Travel Time: Always looking to expand around the globe, the ATP is adding a new stop on its world tour: Johannesburg. The men's tennis tour announced on Thursday that events will be held in the South African city in 2009, 2010 and 2011. The ATP calendar will include tournaments in Shanghai, Tokyo and Beijing after the U.S. Open, followed by a European indoor schedule that ends with the ATP World Tour finals in London. The goal, the tour said, is to cut down on player travel across the continents. "With an enhanced calendar schedule, increased prize money and new world class stadiums this will be huge, positive change that will benefit all of us," ATP board member Ivan Ljubicic said.
Monica Mania: Nine-time Grand Slam champion Monica Seles heads the list of five candidates for the next class in the International Tennis Hall of Fame.
Former French Open winner Andres Gimeno and tennis leaders Donald Dell, Dr. Robert Johnson and Eiichi Kawatei also were put on the ballot on Thursday.
Voting will be held for the next few months and results will be announced in January. The induction is set for July 11 in Newport, Rhode Island.
The 34-year-old Seles won four Australian Opens, three French Opens and twice at the U.S. Open. Known for shrieking when she made shots, Seles finished as the world's No 1 player in 1991 and 1992. She was 19 when a man came from the stands during a 1993 match in Germany and stabbed her in the back. Seles was out of action for more than two years, and later won the Australian Open. Gimeno was a star in Spain during the 1960s and was nominated in the Master Player category. His win at Roland Garros in 1972 made him the oldest French Open champion at 34.
Dell was on the US Davis Cup team from 1961-64, and made his biggest impact on tennis' marketing and business side. In 1972, he and Jack Kramer founded the Association for Tennis Professionals. Johnson helped forge the careers of Arthur Ashe, Althea Gibson and many other black players and pioneered the integration of the sport at the junior level.
Kawatei was nominated for his support and contributions to tennis in Asia and was tournament director for the Japan Open from 1977-86.