WHAT HAPPENED: CoCo Vandeweghe dominated the critical moments to edge past Lucie Safarova, 6-4, 7-6, in Arthur Ashe Stadium and reach her first US Open quarterfinal.

Both sets were tight throughout, but Vandeweghe stepped up when it mattered most:

"I knew in the stickier points of the second set where it was getting tight that I had to show her that I was going to hustle down a few more extra balls," she said. "I was going to be present inside the court for her short balls and not let her dictate."

After holding at love to open the match, the No. 20 seed fought off break points in three of her next four service games to force Safarova to serve to stay in the set.

play video Vandeweghe vs. Safarova

The Czech was finally broken in the 10th game, when a fearless Vandeweghe pressured her into three errors to take the set.

Vandeweghe's advantage slipped early in the second set, as she dropped serve for the only time in the match to fall behind 2-3. But the American quickly regrouped to level the set. Neither player could create any seperation from there, and a tiebreak would decide the set.

In the tiebreak, the Czech started strong, forcing her opponent into two baseline errors for an early mini-break at 2-1. 

Those would be the last points of the match for Safarova, as Vandeweghe again raised her game in the clutch while her opponent seemed to tire.

"I think I just picked up more my pace," the American said.

She quickly reeled off six straight points, capped with a booming ace out wide on the duece court, to seal the victory. 

WHAT IT MEANS: Vandeweghe continues her stellar play at the Grand Slams in 2017. After reaching only one quarterfinal in her first 22 major appearances, she has now reached the last eight for the third time this year after a semifinal run in Australia and a quarterfinal showing at Wimbledon.

One of five Americans to reach Round 4, Vandeweghe becomes the third to punch her ticket to the last eight, with Madison Keys still to play Monday night.

The other American, Jennifer Brady, fell to world No. 1 Karolina Pliskova earlier in Arthur Ashe Stadium. Vandeweghe will have a chance to avenge her compatriot's loss when she takes on Pliskova in the quarterfinals.

MATCH POINT:  Vandeweghe came into the US Open leading the WTA with a 73.8 percent winning rate on first serves. She nearly matched that number today, winning 31 of 45 first serve points, good for 69 percent.