No. 8 seed Li Na enjoyed a comfortable passage into the last 16 of the Mutua Madrid Open on Monday after defeating Spaniard Silvia Soler Espinosa in straight sets 6-3, 6-1.

Li Na showed the power of her serve and looked stronger all round than her rival and broke the Spaniard twice in the first set.

The decisive moment came in the fifth game when she broke Soler 's serve for the first time and then held her own service to love to set up a platform for victory.

She got off to a poor start in the second set as Soler Espinosa broke her serve, but Li Na then produced her full range of shots to win the next six games and end the match in just over an hour.

She will next play German No. 12 seed Angelique Kerber, who beat her this year in Indian Wells.

In the morning session, former world No. 1 Serena Williams grabbed a comfortable 6-3, 6-1 win against Elena Vesnina.

Williams said she still found it easy to find motivated to continue playing aged 30.

"I love playing tennis," she said. "I love walking on court and have people coming to see me play. It is still a thrill and I see it as the ultimate compliment that I have a talent that people come to watch."

No. 3 seed and reigning women's champion Petra Kvitova enjoyed an easy debut in Madrid to defeat Marina Erakovic in 6-2, 6-2.

No. 15 seed Jelena Jankovic suffered an upset, crashing out 4-6, 7-6(5), 6-4 to Spain's Carla Suarez.

Meanwhile China's Peng Shuai lost a long match against the Czech qualifier Lucie Hradecka 6-4, 5-7, 6-4.

In the men's competition, former winner Juan Carlos Ferrero suffered a surprise defeat to Russian qualifier Igor Andreev, although his fellow countryman, the No. 11 seed, Nicolas Almagro had a hard-fought three set win over Colombian Santiago Giraldo in 6-7(5), 6-3, 6-3.