WIMBLEDON, England (AP) — Top-ranked Rafael Nadal rallied for another five-set victory Saturday, overcoming knee problems, an umpire's warning and a tough challenge from a hard-hitting German to reach the fourth round of Wimbledon.

Nadal battled back from two-sets-to-one down for the second straight match to defeat Philipp Petzschner, 6-4, 4-6, 6-7 (5), 6-2, 6-3 in a Centre Court slugfest.

Defending women's champion Serena Williams had a much easier time, serving 19 aces in a straight-sets win over Dominika Cibulkova that set up a fourth-round matchup with former champion Maria Sharapova.

The top-seeded Williams had at least two aces in each of her nine service games and overpowered the 46th-ranked Slovak 6-0, 7-5.

The second-seeded Nadal, who won the title in 2008 but missed last year's tournament due to tendinitis in his knees, called for the trainer four separate times for treatment on his left arm and right knee but never looked badly hurt.

Nadal said he will definitely play Monday's match against France's Paul-Henri Matthieu, who beat Thiemo de Bakker in four sets.

Fourth-seeded Andy Murray followed Nadal on Centre Court and beat Frenchman Gilles Simon 6-1, 6-4, 6-4, keeping alive Britain's hopes for a first homegrown male champion since 1936.

Murray, who hasn't dropped a set so far, served 15 aces, conceded only 15 points on serve and did not face a single break point.

Murray will now face 18th-seeded American Sam Querrey, who overcame Belgian Xavier Malisse to reach the fourth round here for the first time.

The 33rd-seeded Petzschner, playing his third straight five-setter, also needed medical treatment for a recurring hip problem on several changeovers and looked exhausted in the final set against Nadal.

The German questioned Nadal's injury breaks.

Sharapova served an ace on match point to beat Czech player Barbora Zahlavova Strycova 7-5, 6-3. It was an uneven performance by Sharapova, who overcame 35 unforced errors and six double faults.