German former world number one unimpressed by the lack of consistent challengers on the men’s tour.
Roger Federer and Andy Murray advanced to the semi-finals of the Western & Southern Open in contrasting fashion 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.
In the ladies event, defending champion Serena Williams reached the semi-finals but not before having her mettle tested in a 3-6 6-4 6-2 win over sixth-seeded Serb Ana Ivanovic.
Williams, who will be chasing a calendar sweep of the four grand slams at the US Open, struggled with her serve and was forced to come from behind several times in a rematch of last year’s final.
“Out of the previous nine times we’ve played I think we’ve gone three sets at least six times so I know she plays really well,” the top-seeded American said in an on-court interview.
“She’s been number one, she’s won a grand slam. I knew it was going to be a big test for me today and I’m really happy I was able to get through.”
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.