Djokovic and Nadal heading for showdown; Serena marches on in Rome Novak Djokovic reaches semifinal of Rome Masters beating Jo-Wilfried Tsonga Rafael Nadal battles past Tomas Berdych in straight sets Serena Williams into semifinals of accompanying WTA tournament Williams to play Li Na of China for a place in Sunday's final (CNN) -- Novak Djokovic and Rafael Nadal stayed on course for another final showdown in Rome as they conjured up straights sets victories Friday to reach the last four of the Masters 1000 tournament. Defending champion Djokovic saw off fifth seed Jo-Wilfried Tsonga 7-5 6-1 in their quarterfinal match at the Foro Italico, producing some of his best tennis of the year in the second set. The first had been evenly fought with five breaks of service but once the world number one edged ahead after an hour he took command. "In the second set I played one of the best sets on clay this year," he told the official tournament website. This is encouraging for the challenges ahead. With Roland Garros coming, I'm playing and feeling better in every match. Second seed Nadal, who suffered a rare clay court defeat when losing to Djokovic in the 2011 final in Rome, battled past dangerous Czech Tomas Berdych. Nadal was beating the seventh seed for the 11th time in a row, but was pushed all the way in a match lasting a shade over two hours. Three breaks of service and 34 winners for the Spaniard earned him a semifinal match against compatriot David Ferrer, who beat French 15th seed Richard Gasquet 7-6 6-3. "I played one of my best matches on clay against a difficult opponent," Nadal said. Djokovic will next face Roger Federer or home hope Andreas Seppi, who were playing a night match Friday. In the WTA tournament, ninth seed Serena Williams had an easy passage to the last four when her Italian opponent Flavia Pennetta retired with a wrist injury when trailing 4-0 in the opener. Williams won the Madrid title last week and next faces French Open champion Li Na of China. Li, returning to her best form on clay, was rarely troubled by Dominika Cibulkova of Slovakia, winning 6-1 7-6. Defending champion Maria Sharapova continued her title defense in style by beating Serena's sister Venus 6-4 6-3 in their quarterfinal clash. Williams has shown encouraging signs on yet another comeback after illness, but the Russian was the sharper at the business end of each set. She will play either Wimbledon champion Petra Kvitova or Angelique Kerber who were playing in the evening session.