Roger Federer claimed his 50th win of 2014 on Tuesday as the 17-time major winner got his campaign for a record sixth US Open off to a battling start.
The great Swiss, playing in his 60th successive Grand Slam event and bidding to become the oldest major winner in over 40 years, defeated Australia's Marinko Matosevic 6-3, 6-4, 7-6 (7/4) to register his 23rd win in 24 night matches in New York.
"It was a bit more difficult than I thought it was going to be in the third set," said Federer who had been cruising with a break in the third before Matosevic briefly rallied.
Federer next faces another Australian, big-serving Sam Groth for a place in the last 32.
"Marinko has got a big game. He serves and volleys so it gave me a chance to hit some good passing shots."
Second seeded Federer, who hit 10 aces and 41 winners and was cheered on by basketball legend Michael Jordan, found himself sharing the headlines on the second day at Flushing Meadows with teenage upstarts CiCi Bellis and Borna Coric.
Bellis, ranked at a lowly 1,208 and aged just 15, caused the biggest upset when she knocked out Australian Open runner-up, Dominika Cibulkova, the 12th seed, 6-1, 4-6, 6-4 to become the youngest player to win a match at the US Open since Anna Kournikova in 1996.
"I went into the match thinking it was going to be such a great experience, but I never thought I would come out on top winning," said Bellis who, as an amateur, had to pass up the $60,000 prize money to avoid jeopardising a college sports career.
Coric, the 17-year-old world number 204 who was junior champion in 2013, also made a stunning Grand Slam debut with a convincing 6-4, 6-1, 6-2 win over 29th-seeded Czech Lukas Rosol, the man who dumped Rafael Nadal out of Wimbledon two years ago.
The upset put Coric into a second-round clash with history-making Victor Estrella Burgos, the Dominican Republic's first Grand Slam participant who is making his US Open debut at 34.
"It's an amazing feeling," said qualifier Coric, who arrived in New York with the goal of making it into the main draw. "I said to myself, 'If I achieve that, that's going to be perfect'."
- Williams closes programme -
The third-seeded Czech didn't surrender a service break in the 54-minute contest, breaking Mladenovic five times.
Canadian seventh seed Eugenie Bouchard, the Wimbledon runner-up, eased past Olga Govortsova of Belarus 6-2, 6-1, slamming the brakes on her post-All England Club final slump which had brought her just one win coming into New York.
Bouchard is one of four Canadians in the main draw this year, the most since 1989 and is aiming to keep up her record of having made at least the semi-finals at all the majors in 2014.
Former French Open champion Ana Ivanovic, the eighth seed, eased past Alison Riske of the United States, 6-3, 6-0 in a tie which featured eight breaks of serve.
World number one Serena Williams, the double defending champion who is seeking a sixth title, completes the Tuesday programme against 18-year-old compatriot Taylor Townsend.
Williams, an 17-time major winner, has failed to make it past the fourth round at the Australian Open, French Open and Wimbledon this year.
Australia's Samantha Stosur, the 2011 champion, went through with a 6-1, 6-4 win over Lauren Davies of the United States, firmly erasing the nightmare of her 2013 first round exit to US teenager Victoria Duval.
However, another former champion, Svetlana Kuznetsova, the 2004 winner was dumped out 3-6, 6-2, 7-6 (7/3) by New Zealand's Marina Erakovic who won her first US Open match at the fifth time of asking.
Victoria Azarenka, the runner-up to Williams for the last two years, defeated Japan's Misaki Doi 6-7 (3/7), 6-4, 6-1 as the 16th seed looks to resurrect a season wrecked by a left foot injury which restricted her to one event in six months in the first half of 2014.
Two-time men's semi-finalist David Ferrer, the Spanish fourth seed, was also a Tuesday winner, beating Bosnia's Damir Dzumhur 6-1, 6-2, 2-6, 6-2.
America's top hope, 13th seeded John Isner saw off compatriot Marcos Giron 7-6 (7/5), 6-2, 7-6 (7/2) as he bids to end the United States' 11-year wait for a men's Grand Slam title.