STUTTGART - Defending champion Maria Sharapova reached the last eight of the Stuttgart WTA tournament on Thursday with a convincing 6-4, 6-3 win over fellow-Russian Anastasia Pavlyuchenkova. The 27-year-old Sharapova needed just one hour, 32 minutes to see off Pavlyuchenkova to pick up her tenth straight win in Stuttgart, having won the 2012 and 2013 titles, and faces Polish top seed Agnieszka Radwanska in Friday's quarter-finals."I'm pleased to have won in two sets, but I am never really happy with my performance and always want to improve," said Sharapova. Her victory was in stark contrast to her struggles of Tuesday when she needed three tie-breaks and almost three-and-a-half hours to defeat Lucie Safarova of the Czech Republic in her first-round tie.Seeded sixth this year, Sharapova is aiming for a third straight title on the red clay of Stuttgart as the preparations for the French Open get fully underway. The four-time Grand Slam champion looked solid enough on serve, with just five double-faults to eight by her opponent and she had Pavlyuchenkova under constant pressure with her booming returns.Her opponent in the quarter-finals will be Polish top seed Agnieszka Radwanska who proved too good for Italian doubles specialist Roberta Vinci, winning 6-3, 6-2. "Radwanska is the number three in the world and everyone knows she plays extremely well, but I love challenges like this," said Sharapova, who leads Radwanska 8-2 in previous meetings."There are no secrets to our games, we know each other very well, but have not played each other for a while on clay. It will be interesting and I'll have to improve in all areas." Former world number one and Stuttgart champion Jelena Jankovic booked her quarter-final berth as she beat Flavia Pennetta of Italy 3-6, 6-2, 6-3, while Serb ninth seed Ana Ivanovic beat Germany's Julia Goerges 1-6, 6-2, 6-3. "At the end I missed the power in my game, but the positive thing is that I was able to play some good tennis despite all the travel," said 2011 Stuttgart champion Goerges, who had played in Germany's Fed Cup semi-final win in Australia last weekend.