Serena Williams beats Sloane Stephens at Madrid Open

Serena Williams
Williams' last defeat came against Simona Halep in October

World number one Serena Williams extended her winning streak to 25 matches as she beat Sloane Stephens in the second round of the Madrid Open.

Williams, 33, saw off her fellow American 6-4 6-0 to rack up her 25th win stretching back to last season and take her record for 2015 to 22-0.

The top seed will next play Victoria Azarenka or Ajla Tomljanovic.

Fifth seed Caroline Wozniacki and ninth seed Agnieszka Radwanska both won to set up a third-round meeting.

Denmark's Wozniacki beat Christina McHale of the United States 7-5 6-0, while Pole Radwanska defeated Australian Casey Dellacqua 6-2 6-1.

"We grew up playing together," Wozniacki told the WTA.external-link

Williams' winning streak
Serena Williams (three times), Venus Williams, Justine Henin and Victoria Azarenka are the only women to have won 25 or more matches in a row since the turn of the Millennium.

"We've known each other since we were both eight years old. There are no secrets out there - we practise together all the time, and we play against each other a lot too."

Williams took her record against Stephens to 4-1 and said: "She's such a tough player - I've never won a 6-0 set against her before, and I don't think I can ever do that again!

"But today I thought to myself, 'Wow, this is one of the reasons I love this surface.' It just gives you so many chances to get in the point and play yourself into the match.

"I really love playing on the clay."

In the men's draw, Spanish 11th seed Feliciano Lopez and Americans John Isner, Sam Querrey and Steve Johnson all progressed.

Agnieszka Radwanska
Radwanska has slipped from sixth in the rankings at the start of the year to ninth