BEIJING (AP) -- Rafael Nadal is back in form on the tennis court - and in his sharp-tongued critiques of the ATP tour.
The second-seeded Spaniard reached his first quarterfinal since capturing his record ninth French Open title in June with a 6-3, 6-4 win over German qualifier Peter Gojowczyk at the China Open on Thursday, then blasted the choice of tennis balls at the tournament.
''The ball is so bad here,'' Nadal said. ''If you throw the ball on the floor, the bounce goes everywhere. Is not a question of winning or losing. I won in Rio (earlier this year) with this ball.
''It's just that we're competing at the top level of our sport, tennis, and the ball is an important thing.''
This isn't the first time the 14-time major winner has complained previously about the ball in question - manufactured by Head. He also criticized the decision to switch ball manufacturers from tournament to tournament, saying it could result in injuries.
''This week we are playing with one ball. Next week we are playing with a different ball,'' he said. ''That's dangerous for the shoulder, dangerous for the elbow.''
Head is a corporate sponsor of the ATP tour, but each tournament has the right to choose the ball it wants to use from an approved list.
Andy Murray echoed Nadal's complaints about the difficulty of adjusting to a new ball each week.
''If you want to see consistently high level tennis, it's very difficult when you're changing balls from week to week -because they all react differently,'' he said after his 6-2, 6-2 win over Pablo Cuevas.
''I'm sure if you gave golfers a different ball to play with each week, it would take them time to adjust to that, as well.''
Nadal wasn't nearly as critical of his own game in his second match back from a two-month layoff due to a right wrist injury.
He was sharp for much of the match until the final game when he double-faulted and made two unforced errors to give Gojowczyk three break points. He fought them off, however, and closed it out on his fourth match point.
Nadal faces another qualifier in the quarterfinals, Martin Klizan, who advanced when seventh-seeded Ernests Gulbis of Latvia retired with a shoulder injury while trailing 6-2, 3-0.
Murray takes on U.S. Open champion Marin Cilic, a 6-3, 6-3 winner over Portugal's Joao Sousa.
In the women's draw, top-ranked Serena Williams recovered from a second-set lapse to defeat Lucie Safarova, 6-1, 1-6, 6-2.
After the match, Williams skipped her post-match news conference, releasing a statement saying she had ''a little swelling'' in her knee. She wore strapping on the knee during the match she said was preventative.
Simona Halep of Romania won a back-and-forth match against Andrea Petkovic of Germany 7-6 (4), 5-7, 7-6 (1), then immediately withdrew from the tournament with a left hip injury.
The second-seeded Halep took a medical timeout for treatment on her hip after the first set, but returned to complete the match, which featured 17 breaks of serve and 105 combined unforced errors (61 from the Romanian).
''It was a tough match against Andrea today, and I had to fight a lot. I'm happy I was able to win and finish the match in the right way, but I have a hip injury and it would be risky for me to play again tomorrow,'' Halep said in a statement.
Halep's withdrawal gave ninth-seeded Ana Ivanovic of Serbia a pass to the semifinals. The 2008 French Open champion, who is fighting for a spot in the WTA finals in Singapore, earlier beat Sabine Lisicki of Germany 6-3, 7-5.
Two-time Wimbledon champion Petra Kvitova also advanced when Venus Williams withdrew before their match with a virus. Her illness came days after sister Serena retired from a match at last week's Wuhan Open with a virus, as well.
Seventh-seeded Angelique Kerber of Germany was defeated by two-time major winner Svetlana Kuznetsova 2-6, 6-4, 6-3, while No. 12-seeded Ekaterina Makarova of Russia fell to Roberta Vinci of Italy 6-1, 0-6, 7-5.
Other men's winners included third-seeded Tomas Berdych and fifth-seeded Grigor Dimitrov.