U.S. Open Day 8: Serena, Djokovic, Murray into quarters; Bouchard, Raonic ousted

We're on to Day 8 at the U.S. Open - time flies, doesn't it? - in Flushing Meadows, N.Y., and here's what you need to know:

Milos Raonic upset by Kei Nishikori

No. 5 Milos Raonic couldn't advance to his first U.S Open quarterfinal Monday, after No. 10 Kei Nishikori outlasted him in 5 sets, in a match that tied the record for the latest finish in U.S. Open history. Raonic, who won the first set 6-4, couldn't build on the early momentum, losing 7-6 in the second set. He came back with a 7-6 third set before dropping the next two, 7-5 and 6-4. The match lasted over four hours and ended at 2:26 a.m. ET.

Nishikori will now face No. 3 Stan Wawrinka in the quarterfinals Wednesday.

Serena easily into quarters, will face Pennetta

World No. 1 Serena Williams did it:

Williams beat Kaia Kanepi in straight sets, 6-3, 6-3, to secure her first quarterfinals appearance at a Grand Slam this year. She was ousted in the fourth round at the Australian Open, the second round at the French Open, and the third round at Wimbledon. Hard to believe, we know.

Williams will face No. 11 Flavia Pennetta in the next round, after Pennetta beat No. 29 Casey Dellacqua 7-5, 6-2 in her fourth-round match.

Williams is 5-0 lifetime versus Pennetta.

Djokovic, Murray, Wawrinka advance; Bouchard ousted

Men's No. 1 Novak Djokovic won his fourth-round match Monday against No. 22 Philipp Kohlschreiber in straight sets, 6-1, 7-5, 6-4. The Serbian seems to be over whatever was ailing him in early August, when he lost in the Round of 16 at Masters events in Toronto and Cincinnati. 

The quarterfinals appearance is Djokovic's eighth straight at the U.S. Open, and his 22nd consecutive at a Grand Slam. He's good. 

In other men's action, No. 8 Andy Murray beat No. 9 Jo-Wilfried Tsonga in straight sets Monday afternoon, 7-5, 7-5, 6-4. 

No. 3 Stan Wawrinka needed four sets to defeat No. 16 Tommy Robredo (7-5, 4-6, 7-6, 6-2).

On the women's side, another upset: No. 7 Eugenie Bouchard lost to No. 17 Ekaterina Makarova in straight sets, 7-6, 6-4. Bouchard's tournament is over, and she struggled with the heat on a hot day in Queens, N.Y. She took a medical timeout after game five of the second set, as she clearly struggled to stay cool.

Bouchard made 31 unforced errors, won only two of 10 break points, and failed to advance to the quarterfinals at a Grand Slam for the first time this year. It's a disappointing end to Bouchard's Grand Slam season, but she made impressive strides in 2014. 

Meanwhile, there's seemingly unprecedented parity in women's tennis, at least at Grand Slams: 

Azarenka survives after losing first set to Krunic

Victoria Azarenka rebounded after dropping the opening set against Aleksandra Krunic on Monday night, rallying for a 4-6, 6-4, 6-4 victory.

The 16th-seeded Azarenka committed six double faults and landed no aces, but managed a 62% first-serve rate compared to 45% for Krunic.

Azarenka will now face Makarova in the quarterfinals.