NEW YORK – After a thrilling opening week in Flushing Meadows, it seemed as if nothing could top it. Then Monday came and things got off to an auspicious start when Venus Williams and Karolina Pliskova took to the court.

In what will arguably be known as the most exhilarating match at this year's US Open, Venus and Pliskova, who had met just once before, put on a tennis clinic.

Venus roared out of the gate with a 6-4 opening set win before Pliskova countered with a 6-4 second set win of her own. That set up a third and deciding set that lasted 67 minutes with each woman putting the other on the ropes, so to speak, with respective match points. Neither the American nor the Czech gave an inch. Each exchanged eight aces during the match, and were comparative with winners (38-33 in favor of Venus) and total points won (107 to 106 for Pliskova).

In the deciding set, Pliskova held a 4-2 lead until Venus clawed back to make it 5-4 but after forcing a backhand error from the American, the unfazed Czech staved off elimination. The two then exchanged points before facing off in a winner-take-all tiebreak when again, Pliskova jumped out to a big lead. This time, though, it would be a lead that the 24-year-old would not relinquish, holding off Venus 7-3 in the breaker to take the game, set and match, advancing to her first-career Grand Slam quarterfinal.

While advancing to the quarters for the first time is special in itself, it has to be especially so knowing what it took to get there as Pliskova fought back to not only contend against but to eliminate one of the best women's tennis players to ever grace a court. It was Pliskova’s first-career win against Venus, avenging her loss to her in Zhuhai, China, last year.

As thrilling as this win was, though, Pliskova cannot become complacent or overconfident. Waiting for her in the Round of 8 will be 18-year-old Ana Konjuh of Croatia who pulled off an upset of her own in the fourth round, ousting No. 4 Agnieska Radwanska 6-4, 6-4.

With her victory, Konjuh, who is ranked No. 92 in the world, became the youngest US Open quarterfinalist in a decade. So, while on paper, she is an underdog against Pliskova, she was also expected to fall to Radwanska, as well. It will be the first-ever meeting between the two.

On the men's side, dark horse Ivo Karlovic was hoping to reach his first Grand Slam quarterfinal since 2009, but he was up against the former finalist, No. 7 Kei Nishikori, in the fourth round.

The two met three times entering Monday's match with the lanky Croatian coming away victorious twice. However, Nishikori won the most recent meeting and on this day, he would find himself in the win column again, defeating Karlovic 6-3, 6-4, 7-6(4) to advance to the Round of 8.

After falling in the opening two sets in 25 and 40 minutes respectively, Karlovic made Nishikori earn his victory as the Japanese star needed 56 minutes to take the third and decisive set.

Karlovic fired 21 aces but also gifted 23 unforced errors while Nishikori committed just seven. In addition, Nishikori won 84 percent of his first serves and 85 percent of his net points before cementing the win with a forced backhand error.

Nishikori now advances to his fifth Grand Slam quarterfinal and first since the 2015 French Open. He made the Final at the US Open in 2014 and would love nothing more than to return there. However, he must get past No. 2 Andy Murray first and considering the year he's had, which includes finals appearances in each of the three previous Grand Slams, it will be a tall task for Nishikori. It also doesn't help that Murray is coming off winning Wimbledon and being 7-1 lifetime against Nishikori.

Then again, stranger things have happened – and you don't have to look beyond this year's US Open to believe that.