Defending champion and world number one Amelie Mauresmo was overpowered by fifth-ranked Russian Nadia Petrova 6-2 6-2 in her opening round-robin match at the WTA championships in Madrid on Tuesday.

By contrast world number two Maria Sharapova continued her irresistible form with an emphatic 6-1 6-4 victory over fellow Russian Elena Dementieva to extend her winning streak to 17 matches.

Mauresmo, who with Sharapova and Justine Henin-Hardenne is involved in a three-way battle for the year-end number one spot, had been out of action for 10 days with a shoulder problem and looked rusty in her match.

"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."

U.S. Open champion Sharapova, who came into Madrid on a three-tournament winning streak, hit top gear almost immediately in her match against Dementieva, racing into a 3-0 lead in the opening set.

She hit the lines consistently as she worked her compatriot around the court and punished her with a succession of powerful two-handed drives as she took the set 6-1.

Dementieva, the only player to have competed in every end-of-season championship since 2000, rallied at the start of the second, holding serve in a marathon third game.

She broke as Sharapova was serving for the match at 5-3 but the 25-year-old, who was plagued by problems on her service throughout, finally crumbled in the next.

Petrova, runner-up to Sharapova in Linz last month, quickly cranked up the pressure on Mauresmo and broke serve in the second on the way to a 3-0 lead.

She used her aggressive play at the net to stop the Frenchwoman finding her rhythm and broke again at 5-2 up, taking the set with a fine running cross-court return.

It was one-way traffic in the second set as Petrova raced into a 5-1 lead and, although Mauresmo broke back in the seventh and saved four match points, the Australian Open and Wimbledon champion eventually succumbed 6-2.

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

Petrova said she had benefited from Mauresmo's lack of match practice, but added she was pleased with her own performance.

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