I've never seen him play, is he any good?
Sergeyev is an offensive counter-puncher, good defender and hardworker.
His main strengths are movement and solid baseline play. Forehand is his better wing (he runs around his beckhand oftenly), using it he can change directions, change the rhythm and occasionally hits some winners. His backhand is more defensive with irregular depth. He has a good, slice 1st serve - not massive in terms of power (average speed about 180km/h) but is effective and good placement. He’s got solid passingshoots.
His weakness is attackable 2nd serve - passive, and often too short.
While making mistakes, he gets angry, screams, and throws his racket.