Troubled by cramps in both the legs in the third set, Yuki fought off pain and his sluggish rival for a 6-2 6-4 6-7 (1) 6-3 win in two hours and 52 minutes as he put India ahead in the first round tie.
In the second singles, a defensive Somdev was locked in an engrossing battle against an offensive Ti Chen, in which both have won two sets apiece and fifth set was tied 7-7 after four hours and 30 minutes of tennis.
Somdev has squandered four match points, including three in the ninth game of the deciding set, so far and Chen has kept his side's hopes alive to level the tie with his fighting spirit.
The match will be completed tomorrow before Rohan Bopanna and Saketh Myneni clash with Hsien-Yin Peng and Hsin-Han Lee in the doubles rubber.
There was hardly any contest in the opening singles as Yuki hit winner after winner with superb control over his shots and even when he suffered the injury and was limping, Yang failed to take advantage.
To his credit, Yuki did not give up and kept fighting and his persistence finally paid off.
What followed in the next match was engrossing tennis as Chen played a solid game. He just outmanoeuvred Somdev. If Somdev approached net, Chen would past him with blistering winners and when the Indian stayed behind, Chen came up with amazingly calculated drop shots.