Novak Djokovic and Marin Cilic moved on to the men's quarterfinals, while sisters Serena and Venus Williams advanced to the women's quarterfinals of the 2015 US Open Tennis at Flushing Meadows.

Djokovic struggled to find his usual rhythm but still managed to run away with a 6-3, 4-6, 6-4, 6-3 victory over 23rd seed Roberto Bautista Agut of Spain on Sunday at the Arthur Ashe Stadium.

The top-seeded Serb did not drop a single set in his first three matches, but Bautista Agut came prepared. The Spaniard took the second set of the match and broke Djokovic's serve four times.

However, Djokovic kept his composure despite his inconsistent play, but he admitted after the match that he was also frustrated by the way he played against Bautista Agut.

"I was upset with myself," Djokovic said in a post-match interview. "I was frustrated with the fact from 4-2 and 15-40 I wasn't able to capitalize on those opportunities and to lose five games in a row. I cannot allow myself to do that. It happens. I'm a player that shows emotions."

Meanwhile, defending champion Marin Cilic of Crotia overcame a shaky performance in the second set to move on to the quarterfinals with a 6-3, 2-6, 7-6, 6-1 win against 27th seed Jeremy Chardy of France.

Cilic hurt his ankle during the second set of the match, which obviously affected his rhythm as he went on to lose the set. But a strong performance in the third set gave him enough confidence to escape with a four-setter win against his upset-conscious opponent.

In the women's side, Serena Williams leaned on her experience to eliminate her 20-year-old compatriot Madison Keys. The top seed booked a spot in the quarterfinals with a 6-3, 6-3 win against Keys.

Keys showed a lot of promise early in the match, but Williams was simply on target. The 33-year-old superstar, who is eyeing her fourth Grand Slam this year, did not face a break point for the entire match.

With the victory, Williams moved one step closer to sweeping all Grand Slam tournaments this year and winning his seventh US Open title.

However, Williams will have to go through her older sister, Venus, who also advanced to the quarterfinals.

Venus Williams showed that she is still a force to reckon with in the tournament, as the 35-year-old cruised to a 6-2, 6-2 win against Anett Kontaveit of Estonia on Sunday.

Complete 2015 US Open Day 7 results and scores are available on the tournament's official website. Match highlights can be seen on this link.