Serena, Sharapova cruise in Madrid, Djokovic withdraws
File photo shows Russia's Maria Sharapova returning the ball to Poland's Agnieszka Radwanska during their quarter-final at the WTA Porsche Grand Prix in Stuttgart, southwest Germany, on April 25, 2014 - by Thomas Kienzle
World number one Williams began the defence of her Madrid Open title with a comfortable 6-2, 6-1 win over Swiss 17-year-old Belinda Bencic while Sharapova was equally untroubled in a 6-1, 6-2 victory over Czech Klara Koukalova.
Djokovic, the world number two who had been targeting Rafael Nadal's number one ranking, was forced to pull out after failing to recover from his ongoing wrist injury.
The six-time Grand Slam champion suffered the injury in losing to Roger Federer in the semi-final of the Monte Carlo Masters a fortnight ago.
"I am sorry for the tournament and the Spanish fans that I have to pull out of the Madrid Open," said the Serb star, the 2011 champion in the Spanish capital, in a statement.
"I have done everything possible to be able to play in Madrid, which is one of the most important events of the year, but, unfortunately, the injury I have in the right wrist has become inflamed again.
"I will now take my time to recover from the injury with the hope of being ready to play in Rome (from next weekend)."
The injury also means that Djokovic is facing a race against time to be fully fit for the French Open, the only major he has yet to win, which gets under way in three weeks' time.
Williams, a 17-time Grand Slam winner, was playing for the first time since suffering a shock defeat to Jana Cepelova in Charleston over a month ago.
But she showed no sign of rustiness to set up a second round meeting with China's Peng Shuai.
- Relentless Williams -
With Swiss compatriot Martina Hingis watching from the stands, Bencic made a bright start and had two break points to lead 2-1 in the first set.
However she was then unable to cope with Williams' powerful groundstrokes as the American raced through the rest of the first set.
Williams was even more relentless as she moved into a 5-0 lead in the second set and, despite some resistance from Bencic to save match points in the sixth game, the two-time Madrid champion went on to seal victory in just under an hour.
Such was her dominance that Williams even racked up 21 consecutive points at one stage on Sunday.
Four-time Grand Slam champion Sharapova made light work of Czech veteran Koukalova.
Sharapova claimed her first title of the year on clay in Stuttgart two weeks ago and continued her return to form after a difficult start to the year to take victory in little over an hour.
"It was a good start for me," said eighth-seeded Sharapova. "Obviously the first match at a new event is always challenging because you don't know what to expect, especially coming off the indoor tournament last week.
"It's always an adjustment to get yourself prepared for the first round. I'm happy that I won in two good sets today."
Last year's finalist in Madrid will next face unseeded American Christina McHale.
It was a more difficult afternoon for fifth seed Petra Kvitova as she needed a final set tie-break to overcome Romania's Sorana Cirstea 6-1, 5-7, 7-6 (7/4).
The former Wimbledon champion looked well on course for the second round after racing through the opening set, but was then broken four times in the second as the world number 27 levelled the match.
Both women were more stable on serve in the decider as it went to a tie-break after just one break apiece and Kvitova eventually had too much to set up a meeting with American Varvara Lepchenko.
There were also wins for 11th seed Ana Ivanovic who overcame Madison Keys in straight sets 6-1, 7-6 (7/4) and Sloane Stephens who beat Romanian qualifier Monica Niculescu 6-2, 3-6, 6-2.
Seventh seeded German Angelique Kerber was trailing French qualifier Caroline Garcia 6-3, 2-0, when she had to retire because of a back injury.