WIMBLEDON, England (AP) -Novak Djokovic is tough to beat at Wimbledon, indoors or out.
Playing the day's first Centre Court match Friday under the retractable roof, defending champion Djokovic rallied past Radek Stepanek 4-6, 6-2, 6-2, 6-2.
When a drizzle delayed the start of the third round, tournament officials decided Djokovic's match should be played inside. An odd spectacle ensued: The roof closed as the sun came out and outside court covers came off.
"I was a little bit surprised, when I saw sunshine, that the roof is closed," Djokovic said. "Obviously they're relying on a forecast that I don't think is very reliable here. But OK."
The No. 1-ranked Djokovic missed opportunities to seize an early lead, failing to convert his first five break-point chances before he lost serve at love to drop the opening set. But he dominated from there, breaking in the first game in each of the final three sets.
Also advancing to the second week was top-ranked Maria Sharapova. She rallied from a break down in the second set to beat Hsieh Su-wei 6-1, 6-4.
Kim Clijsters led 6-3, 4-3 when No. 12 Vera Zvonareva retired struggling with a cough. The unseeded Clijsters, who plans to retire after the U.S. Open, is a four-time Grand Slam seeking her first berth in a Wimbledon final.
"It's a pleasure to be a part of the second week of a Grand Slam, especially at Wimbledon," she said.
No. 3 Agnieszka Radwanska defeated Britain's Heather Watson under the roof, 6-0, 6-2.
Djokovic has the weekend off and said he might squeeze in a round of golf. His fourth-round opponent will be 34th-ranked Viktor Troicki in an all-Serbian match. Troicki beat No. 15-seeded Juan Monaco 7-5, 7-5, 6-3.
Fans were still buzzing about Rafael Nadal's second-round defeat, which ended under the roof Thursday night, when Djokovic stepped onto the same court. His slow start briefly stirred speculation about another upset.
"I was a set down but managed to make the crucial break in the opening game of the second set," Djokovic said. "And then I thought I played really well."
Stepanek, at 33 the oldest man left in the tournament, repeatedly played serve and volley and had Djokovic on his heels in the early going. Djokovic hit an improbable winner that skipped off the top of the net post and still lost the first set.
Then the match quickly swung his way, and the 28th-seeded Stepanek couldn't compete with Djokovic's mistake-free play. The Serb committed just 13 unforced errors in 221 points.
The match remained entertaining even as it became lopsided. One game in the final set lasted 26 points and had Djokovic smiling at Stepanek's unconventional style, which included a belly flop in pursuit of a shot.
"The fourth set from Novak's side was very impressive," Stepanek said. "I was battling until the end, but the fourth set I felt like no matter what I do on the court, he always answers."
The victory made it two in row under the roof for Djokovic.
"I thought I played great," he said. "But, look, this is an outdoor tournament, so I think everybody wants to play when the roof is open."
Perhaps not Sharapova - she was outdoors struggling with her serve on windy Court 1 and sent one shot over the net on the bounce for a double-fault.
Has she done that before?
"I'm sure I have," she said. "A lot in practice."
The misfire came on break point and put her behind 2-3 in the second set. She rallied by sweeping the final four games.
Sharapova finished with five double-faults but committed only seven unforced errors on her groundstrokes.
"Considering the conditions, I'm pretty happy with the way I played," she said.
Sharapova won the French Open this month to complete a career Grand Slam title. She won Wimbledon at age 17 in 2004 and has reached the final at three of the past four majors.
The half of the men's draw opposite Djokovic opened up when two-time champion Nadal lost to No. 100-ranked Lukas Rosol of the Czech Republic, 6-7 (9), 6-4, 6-4, 2-6, 6-4.
Nadal had reached the past five Grand Slam finals and won his seventh French Open title this month. He also had reached the final in his past five Wimbledons, winning the title in 2008 and 2010.
His departure creates an opportunity for three-time Wimbledon finalist Andy Roddick and Andy Murray, who seeks to become the tournament's first British champion since 1936. Both are on Nadal's side of the draw.