With her No. 1 ranking secure for at least three more weeks, Caroline Wozniacki's taking a break.
Her enforced vacation began Monday when she lost in the fourth round at the Sony Ericsson Open to Andrea Petkovic of Germany, 7-5, 3-6, 6-3.
''I'm just going to take a few days of rest now,'' Wozniacki said. ''I need that, you know – maybe on the beach, get some of the tan lines off.''
Wozniacki's play paled next to Petkovic's at pivotal moments. Leading 5-4 in the opening set, Wozniacki failed to convert three set points. As the final set slipped away, she angrily kicked at a ball – and failed to make solid contact.
That was one of her many unforced errors.
The ranking system draws criticism because Wozniacki has been on top for nearly six months, despite never winning a Grand Slam tournament. She'll remain No. 1 through at least April 18, but with Serena and Venus Williams skipping Key Biscayne for health reasons, Wozniacki missed a chance to strengthen her claim to the top spot.
''I'm not thinking about that,'' she said. ''Of course I would have loved to have won today, but it didn't happen. I'm not going to dig a hole for myself, or dig a grave. I will just take a few days of rest, and I'm prepared for the clay season.''
Petkovic, ranked a career-best 23rd, hit an ace to end the first set and another on match point. She waved her arms as she danced at the net to celebrate her biggest victory.
''It's the Petko Dance,'' she later said with a smile.
No. 16-seeded Maria Sharapova scored her first win over a top-five player in more than three years, beating No. 4 Samantha Stosur 6-4, 6-1. Sharapova, a two-time runner-up at Key Biscayne, earned a berth in the quarterfinals against No. 26 Alexandra Dulgheru.
On the men's side, two-time champion Roger Federer and top-ranked Rafael Nadal advanced to the fourth round. Federer, who slipped to third in the rankings last week, beat No. 32 Juan Monaco 7-6 (4), 6-4. Nadal, seeking his first Key Biscayne title, never faced a break point and defeated fellow Spaniard Feliciano Lopez 6-3, 6-3.
Wozniacki is tied for the tour lead in victories this year, and she earned her 14th career title a week ago at Indian Wells. But she has little history of success at Key Biscayne, where she has yet to reach the semifinals.
Content as always to stay behind the baseline, she doggedly battled Petkovic on a warm afternoon for 2 1/2 hours. But Wozniacki converted only five of 17 break-point opportunities.
''I had so many chances in my first set,'' she said. ''I had set points and I didn't take them. Then the energy level dropped a little bit, and I lost.''
Petkovic became the first German woman to reach the tournament's quarterfinals since Anke Huber in 2001. She'll next face No. 6 Jelena Jankovic, who beat Anabel Medina Garrigues 6-1, 6-3.
Petkovic attributed her upset win to tactics – she varied the pace and occasionally charged forward.
''It was not like I played unbelievable,'' she said. ''It was more that I was able to stick to my game plan and I was playing clever. Most players think they can overpower Caroline. I think that's the wrong approach, because she's going to bring each ball back, so I tried to mix it up a little bit. Thankfully it worked.''
The Petko Dance dates to last year, when she unveiled it after beating Nadia Petrova in the first round of the U.S. Open.
''I wanted to get rid of it after the Open, but the fans said like, `Hey, we are just coming to see the dance, and you're not doing it anymore.' So I brought it back. But this is definitely the last tournament where it's going to happen, and then I'm moving on to something else.''
There was no Sharapova Dance, but lots of fist-waggling from the three-time Grand Slam champion. Her trademark shriek accompanied almost every shot, and the decibel level rose as she closed out the win.
Sharapova has lost only 12 games in three matches after missing Key Biscayne in 2008-10 because of injuries.
''Since I haven't played here in so many years, I feel like I owe it to my fans to play many matches here,'' she told the crowd with a laugh.
She improved to 7-0 against Stosur, who was the French Open runner-up last year but is just 9-8 – all on hardcourts – in 2011.
