Halep has moved from 23rd to No. 19.
Williams, the world No. 1, has won eight tournaments this year while running up a 60-4 record. She enters the U.S. Open this week as the favorite to collect another Grand Slam title.
Halep, with a win Saturday at the New Haven Open in Connecticut, picked up the fourth WTA title of her career -- all four have come since mid-June. Her 6-2, 6-2 win over Petra Kvitova in the championship match was her fifth victory over a Top 10 player. Four of those have come this year.
Halep heads into a first-round match at the U.S. Open against Heather Watson. The loss cost Kvitova, who had been the New Haven defending champion, one spot in the rankings. She's now 10th while Angelique Kerber moves to ninth.
That was the only change in the Top 10 going into the U.S. Open.
Two of the Top 10 will miss the year's final major tournament. No. 3-ranked Maria Sharapova, a semifinalist last year, is out with a shoulder injury. Seventh-ranked Marion Bartoli, a quarterfinalist in New York in 2012, suddenly retired about six weeks after winning her first Grand Slam title at Wimbledon.
The WTA Top 10 heading into the U.S. Open, listing player, home country, ranking points and points won in the 2012 U.S. Open:
1. Serena Williams, United States 12,260 (2,000 champion)
2. Victoria Azarenka, Belarus, 9,505 (1,400, runner-up)
3. Maria Sharapova, Russia, 8,766 (900, semifinalist)
4. Agnieszka Radwanska, Poland, 6,335 (280, fourth round)
5. Sara Errani, Italy, 5,125 (900, semifinalist)
6. Li Na, China, 4,825 (160, third round)
7. Marion Bartoli, France, 4,246 (500, quarterfinalist)
8. Caroline Wozniacki, Denmark, 3,490 (5, first round)
9. Angelique Kerber, Germany, 3,420 (280, fourth round)
10. Petra Kvitova, Czech Republic, 3,290 (280, fourth round)