It was a story of two very different performances from the tennis world's top players on day four of the US Open - where men's number one Rafael Nadal took little time to dispath American Ryler DeHeart, his counterpart in the women's ranking slumped to defeat against Julie Coin, ranked 188th in the world.

Coin (25), in the first Grand Slam tournament of her career, has delivered one of the biggest upsets in history, taking advantage of Ivanovic's mistakes to win the match 6-3, 4-6, 6-3 in just under two hours.

"Today I felt nervous at the beginning and then it went away," Coin said, as quoted by Agence France Press. "I don't know how I did it. I am not thinking about anything right now. Just enjoying the moment."

Ivanovic's mistakes - she had eight double faults and 34 unforced errors to Coin's 27 - helped keep the French qualifier in the match, but she showed resolve half-way through the final set to come back from 0-40 down in the fifth game to hold serve and then break her opponents serve, winning 10 straight points. Ivanovic was left to rue missed opportunities - converting only one of her eight break points in the match and none of four in the deciding set.

The Serbian star, winner of this year's French Open and losing finalist in the Australian Open, came into the tournament looking to improve on her track record at Flushing Meadows. Her fourth-round exit after losing to Venus Williams in 2007 was the furthest she ever progressed at the US Open.

"Obviously if you would ask me at the moment if I'm playing like number one, probably not," Ivanovic said after the match, as quoted by Reuters. "This kind of loss I had today is just incentive to work harder, to go back on the court and to keep working hard and practising and improving."

Coin will meet compatriot Amelie Mauresmo in the third round, with whom she once shared the same coach playing at the same club near Paris, but the two do not know each other well, Associated Press said.
 
Spaniard Nadal, for his part, took just 100 minutes to get past DeHeart, winning in straight sets (6-1, 6-2, 6-4), erasing some of the doubts raised by his underpar performance in beating Germany's Bjorn Phau in the first round.

"I did well. I had to come back in third set from down 3-0 in games so I am happy with the victory," Nadal said, quoted by AFP. "I am happy to be in the third round, for me this is a very important tournament."

Winner at Rolland Garros and Wimbledon this year, Nadal's best previous result at the US Open is a quarterfinal in 2006. His third-round opponent is Viktor Troicki of Serbia. "He is a good player. I am going to have to play better to beat him," Nadal said.

Also on the fourth day, the Williams sisters made easy work of their opponents, Serena winning 6-1, 6-1 against Elena Vesnina of Russia and Venus defeating Rossana De Los Rios 6-0, 6-3. Serena will next play against Japanese veteran Ai Sugiyama, while Venus will take on Ukrainian Alona Bondarenko.

Roger Federer, who lost his top spot in the ATP rankings after holding it for a record 237 straight weeks, will play Brazilian Thiago Alves on day five as he bids to win his fifth consecutive US Open title and his first Grand Slam tournament of the year.