Defending champion and world number one Amelie Mauresmo lost 6-2, 6-2 to fifth-ranked Russian Nadia Petrova in her opening match at the WTA championships on Tuesday.

Maria Sharapova [Images] and Justine Henin-Hardenne, however, the two other players contesting the end-of-year number one spot, notched up impressive wins in their first round-robin matches.

US Open champion Sharapova, who came into Madrid on a three-tournament victorious streak, continued her irresistible form with an emphatic 6-1, 6-4 victory over fellow Russian Elena Dementieva to extend her winning run to 17 matches.

"I don't think I played my best tennis, the first game at any tournament is never easy," Sharapova told reporters. "I hope it is going to get better as things will only get tougher from here on."

Henin-Hardenne, back after a two-month layoff following a calf injury, had to dig deep to clinch a 6-2, 6-7, 6-1 win over Martina Hingis [Images] in the day's most absorbing match.

The Belgian, guaranteed to end the year as number one if she reaches the final, raced through the first set against Hingis and appeared on course for a comfortable win at 5-2 up in the second.

REARGUARD ACTION

But Hingis mounted a furious rearguard action to take the next four games and force a tiebreak which she won 7-5.

Despite her lack of match practice, French Open champion Henin-Hardenne managed to find another gear in the deciding set to ensure a winning return to the court.

Mauresmo, who had been out of action for 10 days with a shoulder problem, looked rusty against Petrova.

"The shoulder didn't hurt me, but the lack of play did," said Mauresmo who had beaten Petrova in five of their previous seven meetings.

"Obviously, it can't get worse and hopefully there is enough time to put things right."

Petrova, runner-up to Sharapova in Linz last month, quickly cranked up the pressure on Mauresmo and had few problems overcoming the Australian Open and Wimbledon champion.

Although she leads the rankings, the only way Mauresmo can claim the year-end top spot is if she defends her title and her rivals slip up badly.

SMART TENNIS

Petrova admitted she had benefited from Mauresmo's lack of match practice.

"It helped me that she wasn't 100 percent, but I played smart tennis, served well and was patient. I felt I was always in control."

Sharapova found top gear almost immediately against Dementieva, racing into a 3-0 lead in the opening set.

The 19-year-old hit the lines consistently as she worked her compatriot around the court and punished her with a succession of powerful two-handed drives to take the set 6-1.

Dementieva, the last player to beat Sharapova before she began her winning streak, rallied at the start of the second, but the 25-year-old, who was plagued by problems on her service throughout, finally crumbled.

"It was very hard to play against her as she is so focused," said Dementieva, the only player to have competed in every end-of-season championship since 2000.

"She is playing like a number one now and I don't think anyone else at the moment can play close to her game," said Dementieva.

• Complete tennis coverage
• All you want to know about Sania Mirza