CINCINNATI - Defending champion Serena Williams reached the semi-finals of the Western & Southern tournament in Cincinnati but not before having her mettle tested in a 3-6 6-4 6-2 win over sixth-seeded Serb Ana Ivanovic on Friday.

Williams, who will be chasing a calendar sweep of the four grand slams at the Aug. 31-September 13 US Open, struggled with her serve and was forced to come from behind several times in a rematch of last year's final. Ivanovic did well to contain her opponent's powerful serve, most notably in the first set when she broke three times in four games to pull ahead 5-3 before delivering an incredible backhand winner to hold to love. A topsy-turvy second set saw Williams grab the first three games and Ivanovic the next four before the world number one roared back to win the next three.

The Serb then grabbed the opening two games of the final set but Williams shifted into a higher gear to win six in a row and advance. The American landed only 31 percent of her first serves in the opening set and 47 percent in the second but was much more efficient in the third, chalking up 68 percent. Up next for Williams will be 14th-seeded Ukrainian Elina Svitolina, a 6-4 2-6 6-0 winner over Czech Lucie Safarova, seeded seven. In Friday's other quarter-finals, third-seeded Romanian Simona Halep dispatched Russian Anastasia Pavlyuchenkova 7-6 6-2. She will now face a semi-final against Serbian Jelena Jankovic who beat Slovakian qualifier Anna Karolina Schmiedlova 6-4 6-2. Halep and Jankovic last squared off in the BNP Paribas Open final in March when Halep prevailed in a three-set battle.

Roger Federer and Andy Murray advanced to the semi-finals of the Western & Southern Open in contrasting fashion on Friday to set up a rematch of their Wimbledon clash. Federer made quick work of Spaniard Feliciano Lopez in a 6-3 6-4 quarter-final romp that lasted just over an hour. Murray needed to grind out another win against Richard Gasquet, outlasting the Frenchman 4-6 6-1 6-4 after two hours.

Third seed Murray saved a match point in the third set and fought fatigue to win his second straight three-set battle. "I fought and gave everything I had, but thankfully it's just been enough in the last two days," Murray, who lost to Federer in straight sets in the Wimbledon semi-finals, told reporters.

Earlier, top seed Novak Djokovic avenged his French Open final loss to Stan Wawrinka by crushing the Swiss 6-4 6-1. Djokovic, now two wins away from becoming the first player to win all nine ATP Masters titles, needed a mere 63 minutes to dispatch fifth seed Wawrinka in the first meeting between the players since Roland Garros.

Wawrinka denied Djokovic a chance to complete a career grand slam in Paris but was wayward in Cincinnati, finishing with 33 unforced errors and pushing a backhand wide on match point. Djokovic, a four-times runner-up in Cincinnati, broke Wawrinka's serve four times and dropped just 12 points on serve in his final tune-up event ahead of the U.S. Open which starts Aug. 31.

"(My performance) came at the right time against one of my biggest rivals and the guy I lost to last time we played in the finals of the French Open," Djokovic said. "Obviously I approached this match very seriously in trying to prepare myself and get myself in a good position to win." The world number one next faces Ukrainian qualifier Alexandr Dolgopolov, a 6-4 6-2 winner over Czech sixth seed Tomas Berdych.