Ndiku began to establish himself as a steeplechaser in the senior ranks in 2011. He competed on the 2011 IAAF Diamond League circuit and set a personal best of 8:07.75 minutes for sixth place at the Herculis meeting – a time which ranked him the ninth fastest in the world that year. He also ranked third at the DN Galan and fifth at the Weltklasse Zürich. He was twice runner-up at the Japanese Corporate Championships. His 2012 season was more low-key and contained few races, with wins at the Hyogo Relays and the Japanese Corporate steeplechase being the highlights. Still, his time of 8:17.88 minutes at the Meeting Areva ranked him in the top 25 for the event. He gained his first senior selection for Kenya at the 2013 IAAF World Cross Country Championships, but failed to finish the race. He completed a Japanese Corporate double over the 5000 m and 3000 m steeplechase, but his season's best time of 8:18.78 minutes was not an improvement on the previous year for the 22-year-old.