New York - Britain, buoyed by the Andy Murray feelgood factor, celebrated US Open first round wins for Laura Robson and qualifier Daniel Evans on Monday as Agnieszka Radwanska and Li Na cruised into the second round.
As the first day of the final Grand Slam of the season got underway at Flushing Meadows, world number 179 Evans, making his debut after coming through qualifying, swept past Nishikori, 6-4, 6-4, 6-2.
Nishikori, who made the third round last year, committed 38 unforced errors.
The 23-year-old Evans had never won a main draw match at the Grand Slams until Monday and his reward is a second-round clash against Australia's Bernard Tomic, who needed five sets to see off Albert Ramos of Spain.
Monday's win guaranteed Evans at least $53,000 -- $10,000 more than he's made in all of 2013 so far.
"I really want to be top 100, so that's when the money will start to come in, is when I'm top 100 consistently," said Evans.
"It is an added bonus. But it would be a bit strange if I was just thinking about the money after I've just beat the 11th seed."
Nishikori admitted the Briton was able to play risk-free.
"He had no pressure and I was tight," said the Asian number one. "I haven't played well these last three weeks but I thought it was getting better."
Venus Williams, the back-to-back champion in 2000 and 2001 and the second oldest woman in the draw at 33, started her 60th Grand Slam and 15th US Open with a 6-1, 6-2 win over Belgian 12th seed Kirsten Flipkens, a semi-finalist at Wimbledon.
Williams, now ranked 60 in the world, faces either China's Zheng Jie or Kiki Bertens after avenging a loss to Flipkens at Toronto earlier this month.
"It's just good to be here, see familiar faces since my first time in 1997 with the beads and the braces," said Williams after firing 21 winners.
The American improved to 15-0 in US Open openers by beating Flipkens.
Polish third seed Agnieszka Radwanska, who has never got beyond the last-16, eased past Silvia Soler-Espinosa 6-1, 6-2 and next faces another Spaniard, Maria Teresa Torro-Flor.
Chinese fifth seed Li Na also reached the second round with a 6-2, 6-2 win over Olga Govortsova of Belarus.
Li, a quarter-finalist in 2009, next plays Sofia Arvidsson of Sweden.
Britain's Laura Robson, the first British woman to be seeded at a major for 26 years, won nine games in succession to beat Spanish veteran Lourdes Dominguez Lino, 7-5, 6-0.
The 30th seed, who ended Kim Clijsters' career in New York 12 months ago on her way to the last 16, next faces France's Caroline Garcia.
"I think I played really well here last year but this is only the second round. There's a long way to go," said the 19-year-old Robson, who had lost both her previous meetings with Dominguez Lino.
Later Monday, second seed Rafael Nadal, the 2010 champion, and Roger Federer, the former five-time champion and seeded a lowly seventh this year, begin their campaigns.
Nadal, who is undefeated on hard courts in 2013 and arrives in New York with back-to-back Masters titles in Montreal and Cincinnati, faces American wildcard Ryan Harrison.
Federer, the 17-time Grand Slam champion, concludes the night session on the showpiece Arthur Ashe Stadium against Slovenia's Grega Zemjla.
Defending women's champion Serena Williams, a four-time winner, kicks off the evening action against Francesca Schiavone, the former French Open winner from Italy.
As well as tennis A-listers, the tournament will also welcome rock royalty with Lenny Kravitz, the Grammy Award winner, one of the warm-up acts at the opening ceremony which also features Oscar-winning actors Kevin Spacey and Cuba Gooding.
Meanwhile, 12 months after 2003 champion Andy Roddick quit the sport, another US veteran, James Blake, announced his retirement on Monday.
The 33-year-old, who reached as high as four in the world back in 2006 but now stands at 100, won 10 tour titles and made the US Open quarter-finals the same year.
"This is my last tournament," said Blake.
"I have had 14 pretty darn good years on tour, loved every minute of it, and I definitely couldn't have asked for a better career."US Open results on Monday at Flushing Meadows (x denotes seeded player):
Richard Gasquet (FRA x8) bt Michael Russell (USA) 6-3, 6-4, 6-2
Stephane Robert (FRA) bt Albano Olivetti (FRA) 6-3, 6-3, 6-4
Guillaume Rufin (FRA) bt Jan-Lennard Struff (GER) 7-6 (7/4), 6-3, 2-6, 2-6, 6-1
Dmitry Tursunov (RUS x32) bt Aljaz Bedene (SLO) 7-5, 4-6, 6-3, 6-0
Feliciano Lopez (ESP x23) bt Florent Serra (FRA) 6-7 (4/7), 6-2, 6-3, 6-3
Bradley Klahn (USA) bt Kenny De Schepper (FRA) 6-7 (5/7), 6-2, 7-6 (7/0), 7-6 (7/4)
Mikhail Kukushkin (KAZ) bt Andrej Martin (SVK) 6-4, 7-6 (7/2), 7-5
Bernard Tomic (AUS) bt Albert Ramos (ESP) 6-3, 3-6, 4-6, 7-6 (7/1), 6-3
Daniel Evans (GBR) bt Kei Nishikori (JPN x11) 6-4, 6-4, 6-2
Yaroslava Shvedova (KAZ) bt Olga Puchkova (RUS) 6-1, 6-0
Patricia Mayr (AUT) bt Magdalena Rybarikova (SVK x29) 7-6 (7/2), 6-3
Jamie Hampton (USA x23) bt Lara Arruabarrena (ESP) 6-4, 6-2
Venus Williams (USA) bt Kirsten Flipkens (BEL x12) 6-1, 6-2
Coco Vandeweghe (USA) bt Aleksandra Krunic (SRB) 6-4, 7-6 (7/5)
Carla Suarez (ESP x18) bt Lauren Davis (USA) 6-0, 6-0
Kaia Kanepi (EST x25) bt Vania King (USA) 4-6, 7-6 (7/4), 6-1
Anna Karolina Schmiedlova (SVK) bt Stefanie Voegele (SUI) 6-1, 5-7, 7-6 (7/4)
Agnieszka Radwanska (POL x3) bt Silvia Soler Espinosa (ESP) 6-1, 6-2
Maria Teresa Torro Flor (ESP) bt Marina Erakovic (NZL) 6-0, 6-4
Anastasia Pavlyuchenkova (RUS x32) bt Virginie Razzano (FRA) 7-5, 6-0
Sabine Lisicki (GER x16) bt Vera Dushevina (RUS) 6-2, 7-6 (7/3)
Laura Robson (GBR x30) bt Lourdes Domnguez (ESP) 7-5, 6-0
Caroline Garcia (FRA) bt Shelby Rogers (USA) 6-3, 6-2
Sofia Arvidsson (SWE) bt Petra Cetkovska (CZE) 1-6, 6-4, 6-1
Li Na (CHN x5) bt Olga Govortsova (BLR) 6-2, 6-2