Kanepi tightened up in the second set, seeing a 3-0 lead wiped out at 4-4, but she kept swinging for the lines and, after breaking Halep for the fifth time, confidently served out one of the wins of her life.
Some players, like top-ranked Simona Halep, freely acknowledge they don't deal well with the hustle-and-bustle of the U.S. Open and all it entails.
But Kanepi ended a 14-stroke exchange with a cross-court forehand volley victor to break right back, then served out the victory. The current match wins leader on tour (46-8), Halep also led the way for hardcourt wins (28-4) but Kanepi, a quarterfinalist a year ago as a qualifier, claimed her first win over a Top 20 opponent since 2015 Madrid.
Kaia Kanepi defeated Halep 6-2, 6-4 on Monday in the first round in NY.
Despite battling back to 4-4 in the second set, Halep wasn't able to hold on.
Frustration boiled over for the Romanian No.1 as she destroyed her racquet early in the second set but she showed great fight to recover both the two breaks she suffered to keep her hopes alive. "I like the city", said Kanepi, who is from Estonia and is sharing a coach this week with another player, Andrea Petkovic.
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But Kanepi completed the upset when Halep sent a service return long on match point.
"Today I just lost", Halep told reporters.
Of the 34 Grand Slam tournaments SHalep has played in, she has gone out in the first round on 12 occasions.
Seeding aside, the last world No. 1 to win the U.S. Open was Serena Williams in 2014. She then reached the final in Cincinnati, making her a favorite at the U.S. Open. Halep, however, has already secured her rank through the tournament, so she will remain No. 1.