Miami, FL (SportsNetwork.com) - Seven-time champion Serena Williams overcame a shaky start to record her 700th career match win Wednesday at the $5.3 million Miami Open tennis event.

A celebratory cake was brought onto the court after the match.

"I had no idea," Williams said of the milestone. "I saw them bringing out a cake and I thought, 'Whose birthday is it today?' But I really had no idea about it, so I'm really excited right now!"

The world No. 1 superstar was tested mightily in a 7-6 (7-4), 1-6, 6-3 decision against 27th-seeded German Sabine Lisicki on the hardcourts at Crandon Park. Williams needed just over two hours to move on, which she did despite having her powerful serve broken six times.

She prevailed despite an unreliable serve, a poor second set and 51 unforced errors.

Williams, however, has now won her last 19 matches on tour and 16 straight matches in Miami.

"I wasn't playing my best today," Williams said afterwards. "But some days you're just off, and that's when you have to fight. Even if I lost today, if I just gave 100 percent then I wouldn't have been upset."

Lisicki stunned Williams in a fourth-round affair at Wimbledon two years ago. The American, however, now leads their lifetime series 3-1.

The incomparable Williams has appeared in nine Miami Open finals, going 7-2 in those title tilts.

The 33-year-old reigning Australian and U.S. Open champ beat since-retired Li Na in last year's finale for a second straight title in "Magic City."

The 19-time Grand Slam titlist will meet red-hot third-seeded Romanian star Simona Halep in Thursday's semifinals. Halep beat American Sloane Stephens by a 6-1, 7-5 margin.

Halep raced out to a 6-1, 3-0 lead, but Stephens won five of the next six games to go up 5-4 in the second set. However, Halep fought back to close out the match in two sets.

"The beginning of the match was really fast. I think I was really playing my best tennis," Halep said in her on-court interview. "She came back and changed the tactics, playing a little bit slower, and I lost my rhythm for a little bit. But I'm really happy I could win the match in two sets in the end."

Last year's French Open runner-up Halep has won her last 15 matches on tour, which includes a title at Indian Wells prior to this Miami fortnight.

Williams and Halep were set to have their first meeting in 2015 in the semifinals of Indian Wells, but Williams was forced to withdraw before the match due to a right knee injury.

The other semi will pit ninth-seeded German Andrea Petkovic against 12th- seeded Spaniard Carla Suarez Navarro.