"Accelerate" (加速, Kasoku) is the 43rd chapter of the One-Punch Man manga series.
Sonic comes down from atop the street light, boasting that he shall beat Genos and Saitama shall be next. Meanwhile, Fubuki notices his killing intent and claims him to be dangerous. Genos tells Saitama that he shall beat Sonic, to which Sonic is amused as he claims Genos to be slow. Genos then responds that he did not show what he is truly capable of. Genos then accelerates himself and quickly moves behind Sonic, unsuccessfully attacking yet again because Sonic manages to catch on and dodge. Saitama and Fubuki witness the match and while Saitama notices Genos moving faster than he did when they fought, even hypothesizing that he got new parts, Fubuki is unable to keep up with the speed of the two.
After a bit of fighting, Sonic notices Genos keeps getting behind him, thinking he is mocking his speed. Genos then drops Sonic's topknot, leaving him with short hair, which angers Sonic.
Genos gets behind Sonic and attacks with his Machine Gun Blow technique, only to be surprised to learn he had only hit an afterimage. Sonic then retaliates with his secret technique: Four Shadows Burial - Scattered Flash Slash, which Genos cannot follow. This prompts Genos to blow up the entire area, going at maximum power. Afterwards as Sonic is thought to have beaten Genos, he realizes Genos had fallen before he was able to cut him. Learning Saitama was responsible, he calls him soft for wanting to protect his pupil, to which Saitama replies it was to prevent Genos from destroying the place, since they live in the vicinity. Saitama then tells Genos to stop his fight as for he himself was Sonic's target, then states he shall take Sonic on seriously. Sonic then states he shall use his ultimate technique he developed, in order to kill Saitama.
Meanwhile, Fubuki is watching the entire fight, and is glad she's ignored for otherwise she might have been in danger.
Sonic attacks Saitama with his ultimate technique: Ten Shadows Burial, to which both Genos and Fubuki are impressed but are still capable of seeing. He does not have the upper hand and Saitama counters with his own super move: Serious Series - Serious Side Hops, creating an infinite amount of clones and beating Sonic. Sonic then appears to be knocked out, which to Genos resembles death, but Sonic managed to instantly leave. Fubuki, seeing everything, claims Saitama to be too strong.