Williams effectively reduced the errors and created the great services when he needed them.
He finished the game with 13 aces and will face Anastasija Sevastova for a place in the final after the sixteenth seed defeated defending champion Sloane Stephens 6-2, 6-3.
Pliskova, who managed to convert just two of 12 break points in the contest, said Williams was simply too good at the important moments.
"She connected all the first services in the break points, which is a bit more difficult," Pliskova said.
"In the second set, she was just mixing the service, I was going too far, there's not much I can do in the break points."
Williams dropped his serve once in the second, coughing one of his five double faults at the break point when Pliskova reduced the gap to 4-1.
But when he dropped below 0-40 in the seventh game he was rescued with a 112 mph (180 km / h) service winner followed by another service winner and an ace.
Pliskova had another chance when Williams landed a forehand on the net, but another booming service, a backhand winner and a non-recoverable service saw her safe through danger.
Serve for the match at 5-3 Williams left no doubt, endured love with two aces and a great finish followed by a last ace.
"I really feel that I am now playing for free because this time I had a baby last year, so I have nothing to prove," said Williams, who is seeking his first major title since his daughter Olympia was born on September 1. of 2017.
With a win, he would break a tie with Chris Evert in most US Open titles, and match Margaret Court's Grand Slams record.
Stephens does not connect
The number three in the world, Stephens, refused to blame the punishment for the heat and humidity of the afternoon for his failures against Sevastova, including the inability to convert any of the seven break opportunities in the first set.
"When you do not play well, the game can get away from you," he said. "Mentally, physically, I was just not connecting."
Sevastova herself relies on variety rather than power, and caught Stephens moving slowly with a series of descending shots and shot-ball combinations.
After taking a 4-1 lead in the second set, Sevastova admitted he had flashbacks to the quarterfinals last year, when Stephens, 83rd, recovered from a break in the third set to upset Sevastova in a tiebreaker.
In fact, Stephens struggled to reduce the difference to 4-3, but Sevastova took another break with a timely pitch for a 5-3 lead and sealed the victory at his third match point when a tired Sephens put a backhand In the net .
"I lost my nerves a little," Sevastova admitted. "I think he also lost a little nerves, it's normal, it's for the semifinals of the US Open."
The defeats of Stephens and Pliskova completed the exodus of the 10 best seeds, though Pliskova noted that Williams' 17th seed – 9 points above her world ranking as she continues her post-baby comeback – did not reflect her true abilities.
"He is 17 years old, but he is not a player who should be 17 years old," Pliskova said.
The semifinal line-up will be completed on Wednesday when Spain's Carla Suárez Navarro faces runner-up 2017 Madison Keys and Japan's Naomi Osaka takes on Ukrainian Lesia Tsurenko.