Lucky number 700 for Serena Williams

World No. 1 Williams looked to be heading for a surprise defeat against Sabine Lisicki as she surrendered six straight games in the second set having won a tight opener.

But the American rallied to overcome the 21st-ranked German 7-6 1-6 6-3 in a little over two hours and celebrated with a hard-earned slice of cake.

"I had no idea," said Williams, who last year won a record seventh Miami title. "I saw them bringing out a cake and I thought, 'Whose birthday is it today?' But I really had no idea about (the milestone)."

A 19-time grand slam winner, Williams has raked in more than $65 million in prize money.

It hasn't all been plain sailing however -- in 2010 doctors found a life-threatening blood clot in her lungs resulting in the star taking a break from competition for 12 months.

Yet to lose a match in 2015, the reigning Australian and U.S. Open champion is writing her name across the history books, although she still has a long way to go to beat Martina Navratilova's record of 1,442 match wins -- the leading mark in both the men's and women's game.

In Thursday's second semifinal, Williams will face world No. 3 Simona Halep, the last player to defeat her in October last year.

Halep became the first Romanian woman to beat a No. 1 player, winning their group match at the WTA Finals, but lost to Williams in the title match in Singapore.

They were supposed to meet again last month at Indian Wells, but Williams pulled out of their semifinal due to a knee problem.

"I'll remember beating her all my life because it was such a good match for me," said Halep, who beat young American Sloane Stephens -- often lauded as "the next Serena" -- in her quarterfinal.

"I have to believe I can win again against her -- I just have to go out there and believe I have a chance.

"I have nothing to lose. She's the best player in the world."

The winner of their match will play either German ninth seed Andrea Petkovic or Spanish No. 12 Carla Suarez-Navarro, who beat Serena's older sister Venus on Tuesday.

(CNN)