German former world number one unimpressed by the lack of consistent challengers on the men’s tour.
Top-seed Rafael Nadal moved into the quarter-finals of the Hamburg Open with a 6-4, 7-6 win over Jiri Vesely.
Nadal appeared in trouble during Thursday’s match when he dropped his serve for 4-4 in the second set.
But he broke right back and served for the match, only to commit a double fault at match point.
The Spaniard served another double fault to drop the game, but Vesely could not take advantage and Nadal cruised through the tiebreaker, winning the match when Vesely netted a backhand.
Nadal is playing in Hamburg for the first time since he won the clay-court event in 2008.
The former world number one has fallen to 10th in the world rankings following injury and poor form.
He was knocked out of Wimbledon in the second round.
The Spaniard will now take on fifth-seed Pablo Cuevas of Uruguay, who outlasted Jerzy Janowicz of Poland 6-3, 4-6, 6-2.
Andreas Seppi of Italy also reached the quarter-finals by beating Florian Mayer of Germany 4-6, 6-2, 6-3.