The top men's and women's seeds have advanced through the first round of the US Open tennis championship in New York.
Novak Djokovic advanced to the second round when his opponent, Conor Niland of Ireland, retired sick while trailing 6-love, 5-1.
Niland, the first Irishman ever to make the main draw of the U.S. Open, had been suffering from food poisoning even before the match began and was unable to muster any real challenge to the tournament favourite.
Women's number one Caroline Wozniacki scored a brisk 6-3 6-1 victory over 125th-ranked Nuria Llagosera Vives of Spain.
The top-seeded Dane remains in search of her first grand slam title.
French Open champion Li Na of China was beaten, following Wimbledon champion Petra Kvitova out in the first round.
That means none of the current women's grand slam champions have made the second round, with the defending US and current Australian champion Kim Clijsters no playing at the tournament because of injury.
Rafael Nadal the defending men's champion had a straight sets win over Andrey Golubev of Kazakhstan, but suffered six service breaks in the process.
Serena Williams was on her best behaviour and close to her best form, when she made her first appearance at the tournament in two years.
The American showed no signs of rustiness or fatigue despite her recent return from a lengthy layoff because of health problems, needing less than an hour to win her opening round match in straight sets, 6-1 6-1.
Her last appearance at the tournament two years ago ended in disgrace when she abused a line judge and was docked a point that cost her the match against eventual champion Kim Clijsters and a hefty fine.
She missed last year's tournament through injury and has been out for most of this season because of health complications.