ROME - Top-ranked Novak Djokovic fought back from a set down to pull out a 5-7, 6-2, 6-3 win over Brazilian qualifier Thomaz Bellucci to reach the Italian Open quarterfinals on Thursday.

The 68th-ranked Bellucci gave Djokovic trouble with his powerful serving and groundstrokes and also exhibited extraordinary foot speed early on.

After Bellucci broke to take a 6-5 lead in the opening set, Djokovic put on a new pair of sneakers during the changeover — a sign he wasn't comfortable.

Djokovic broke early in the second set and got the decisive break late in the third, extending his winning streak to 18 matches.

He will next face fifth-seeded Kei Nishikori, who beat Viktor Troicki of Serbia 6-4, 6-3.

Earlier, Rafael Nadal became the first player to break John Isner's serve in nearly a month, and he did it twice in a 6-4, 6-4 win.

Andy Murray and Serena Williams, meanwhile, withdrew from the last big warm-up tournament before the French Open, which starts in 10 days.

Murray cited fatigue following consecutive clay titles in Munich and Madrid, while Williams said she has a right elbow injury.

Isner held serve in 84 consecutive games stretching back to his last meeting with Nadal on April 16 in the Monte Carlo Masters, when Nadal broke him in the third set and went on to win.

This time, Nadal broke to take a 3-2 lead in the first set with a forehand winner that landed on the line on his first break point. Then the Spaniard produced a whipping forehand return pass up the line off Isner's second serve to go up 5-4 in the second, and quickly closed it out from there.

"When I had chances I (took advantage)," Nadal said. "My serve was perfect the whole match. I played with (few) mistakes."

Nadal will next face eighth-seeded Stan Wawrinka, who beat Dominic Thiem of Austria 7-6 (3), 6-4.

Roger Federer struggled only to close out a 6-3, 7-5 win over big-serving South African Kevin Anderson. Anderson served 14 aces to Federer's four, and matched the 17-time Grand Slam champion with 22 winners, but never really put Federer under pressure.

Federer lined up sixth-seeded Tomas Berdych, who overcame a strong challenge and a partisan crowd for a 6-3, 3-6, 7-6 (2) win over Fabio Fognini, the last remaining Italian. Amid a raucous atmosphere on a secondary court, the match lasted more than 2 1/2 hours.

Also through was seventh-seeded David Ferrer, who defeated Guillermo Garcia-Lopez 6-1, 6-3. Ferrer's quarterfinal opponent will be David Goffin of Belgium, who advanced when Murray pulled out.

In women's action, No. 10 Carla Suarez Navarro defeated Canada's Eugenie Bouchard 6-7 (2), 7-5, 7-6 (7). Bouchard, the No. 6 seed in Rome, has lost seven of her last eight matches over WTA Tour and Fed Cup events.

Second-seeded Simona Halep routed 1999 Rome champion Venus Williams 6-2, 6-1; two-time champ Maria Sharapova eliminated Serbian qualifier Bojana Jovanovski 6-3, 6-3; and Carla Suarez Navarro defeated Eugenie Bouchard 6-7 (2), 7-5, 7-6 (7).

American qualifier Christina McHale made the last eight when Serena Williams withdrew.

In women's doubles, Ottawa's Gabriela Dabrowski and Poland's Alicja Rosolska advanced to the quarter-finals with a 6-4, 4-6, 11-9 win over the sixth-ranked Czech pair of Andrea Hlavackova and Lucie Hradecka. In men's doubles, Toronto's Daniel nestor and Indian partner Leander Paes were fedeated 6-2, 6-4 by Pablo Cuevas and David Marrero.

A seven-time Rome champ, Nadal won a higher percentage of points on his first serve and virtually all of the long rallies against the six-foot-10 American, who has given him trouble in the past.

On Monday, Nadal dropped out of the top five in the ATP rankings for the first time in 10 years — falling to No. 7. Struggling to get back to his best after a wrist injury and an appendectomy last season, Nadal has lost four times on clay this year — something he hasn't done since 2003.

But he remains optimistic.

"It seems like I'm having much more good days than bad days," Nadal said.

Murray dominated Nadal in Sunday's Madrid Open final, a week after taking his first clay title in Munich.

"I'm tired. My body is tired," Murray said. "It's completely normal, and I think acceptable to feel like that after the last few weeks."

Serena Williams is trying not to make the same mistake as last year and let an injury affect her chances at Roland Garros.

"We have to make the right decisions for the future, not for now," Williams said. "You know I hate, hate quitting, and this isn't quitting, it's just making a good decision.

"I was really injured last year, actually, and ended up taking like five days off before Paris and practicing just a day or two before the tournament started," added Williams, who lost in the second round. "And entering a Grand Slam, you never want to enter it like that, especially as defending champion."

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Andrew Dampf can be followed at www.twitter.com/asdampf