FLUSHING MEADOWS, N.Y., Sept. 4 (UPI) -- That was a close call.
Williams started slow out of the gate in her night match at Arthur Ashe Stadium Friday, losing the first set 3-6 to fellow American Bethanie Mattek-Sands -- committing 14 unforced errors to Mattek-Sands' one.
"I just felt I couldn't really get a rhythm," Williams said of the first set after the match. "Knowing that I have another level and I can play better helps."
The six-time U.S. Open champion continued to fight off a hard-charging Mattek-Sands in the second set, as well -- but ultimately evened the match by winning set two, 7-5. Williams put Mattek-Sands away with a dominant third set, winning 6-0.
Williams is four match victories away from her seventh U.S. Open trophy, which would also win her the coveted Grand Slam -- titles at all four major tennis tournaments in the same calendar year. No player has won a calendar Slam since Steffi Graf in 1988.
Next up for Williams is another American, 19th-seeded Madison Keys -- who knocked off 15th-seeded Agnieszka Radwanska (Poland) in straight sets Friday.
"She's really good ... She's on a roll right now. We'll see what happens," Serena said.
Williams' sister Venus also continued to roll at the Open, notching an upset over 12th-seeded Belinda Bencic, 6-3 6-4. If she defeats Estonian Anett Kontaveit on Monday -- and Serena beats Keys -- the sisters will meet in the U.S. Open semifinals.
Meanwhile, men's top seed Novak Djokovic again advanced Friday with another straight-set victory over 25th-seeded Andreas Seppi. The Serbian champion, who's looking for his second title in New York, has yet to drop a set at the tournament so far.
Djokovic will face Spaniard Roberto Bautista Agut, who beat 14th-seeded David Goffin Friday, in the fourth round.
Croatian Marin Cilic, last year's men's champion, was pushed to five sets again during his match Friday -- ultimately defeating Mikhail Kukushkin for a spot in the fourth round.
- 8th-seeded Rafael Nadal played Italy's Fabio Fognini in the men's night match Friday.
- 18th-seeded Spaniard Feliciano Lopez defeated 10-seed Milos Raonic (Canada), 6-2, 7-6, 6-3
- 3-seed Andy Murray (Britain) is once again scheduled for the men's night match Saturday, after needing a furious 2-sets-to-none comeback to beat France's Adrian Mannarino Thursday night.
- 13th-seeded American John Isner will try for a spot in the fourth round against Czech Jiri Vesely on Saturday.
- The only other American men's player remaining, Donald Young, will play Serbian Viktor Troicki Saturday for a spot in the fourth round.
- 21 of 32 seeded men's players remain in the tournament. Just 15 of 32 remain in the women's draw.