Wimbledon 2012: Juan Martín del Potro defeats Kei Nishikori to end Japanese challenge But Nishikori is playing much better than two or three years ago, and he's still playing better and better. So he could be very dangerous for the rest of the players. Nishikori was Del Potro's second Japanese victim at Wimbledon, the 23 year-old having beaten Go Soeda in four sets in the second round. I think it was my best game of the week. Kei Nishikori on grass is always going to be a big challenge," he added. I'm glad to be in the fourth round. I will try to play better and better as I go on, because I want to stay alive in this tournament for a few more days. The highest-placed Japanese man since the rankings began in 1973 had lost in the first round on his three previous Wimbledon appearances but showed his true potential this time. The fifth seed, Jo-Wilfried Tsonga, meanwhile beat Slovakia's Lukas Lacko 6-4, 6-3, 6-3. The fifth-seeded Frenchman never lost his serve and broke Lacko four times to wrap up the victory in one hour and 36 minutes on Court No 1. Tsonga reached the semi-finals at the All England Club last year after rallying from two sets down to beat six-time champion Roger Federer in the quarter-finals. He went on to lose against eventual champion Novak Djokovic. Tsonga will now play American 10th seed Mardy Fish, a quarter-finalist last year and playing his first tournament since undergoing a heart operation, who beat Belgian wild card David Goffin 6-3, 7-6, 7-6. In the men's doubles, the Bryan brothers, Bob and Mike, the second seeds, comfortably defeated Britain's Jamie Delgado and Kenneth Skupski 7-6, 6-0, 6-2.