Nishikori advances, Federer and Nadal in the wings
Japanese Kei Nishikori returns the ball to French Gael Monfils during the ATP Gerry Weber Open tennis tournament in Halle, western Germany on June 11, 2013 - by Carmen Jaspersen
The number four seed, who dropped to world number 12 following his first round exit at the French Open, will next play American Steve Johnson or Russia's Teymuraz Gabashvili for a place in the quarter-finals.
Monfils, who has a poor record on grass, blamed his dislike for the surface on the defeat in just under 90 minutes.
"I'm not good on grass. I give it my best and grit my teeth but it's not natural for me," said the last Frenchman to be eliminated from the German-hosted tournament.
"I'm going to see how I wake up tomorrow," he added, when asked about the rest of the grass court season and a potential withdrawal from Wimbledon.
In other second round matches, Germany's Philipp Kohlschreiber, who fell in five sets against Andy Murray in the fourth round at Roland Garros, advanced easily with a 6-2, 6-4 victory over Frenchman Pierre-Hugues Herbert while Colombian Alejandro Falla knocked out Dutchman Robin Hasse 7-6 (7/1), 6-4.
Kohlschreiber, the 2011 Halle champion who has won four of his five ATP titles in Germany, could play world number one and recently crowned French Open champion Rafael Nadal or Germany's Dustin Brown in the quarter-finals.
Nadal, who has a 50-13 winning record on grass and won Wimbledon in 2008 and 2010, has never been past the quarter-finals at the Gerry Weber Open as he gears up for his third attempt at a tournament dominated by Roger Federer.
The Swiss 17-time Grand Slam winner, is chasing his seventh title at Halle and reached the doubles' quarter-finals on Wednesday with compatriot Marco Chiudinelli, courtesy of a walkover over German Martin Emmrich and Italian Andreas Seppi.
He takes on Portugal's Joao Sousa, ranked world number 47, on Thursday in his first singles' match.
"It's an interesting period because I have my title to defend here, and actually at Wimbledon, I don't have anything to defend," said Federer.
"But I have to prove myself somehow. So, actually it’s the kind of the wrong way round for me, because usually the pressure in Wimbledon is much bigger because of the points and because you want to play well.
"But this year, I might go to Wimbledon a bit more relaxed. The pressure here is quite big, because I want to play well. The draw at the Gerry Weber Open is tough.
"Therefore it's not easy to target the title. But to me it's obvious why I'm here. I want to capture the title and play well. So, that’s absolutely my priority."
Canadian third seed Milos Raonic is in action later on Wednesday as he takes on German wildcard Peter Gojowczyk, ranked 120 in the world.