Japan's world No. 7 Kei Nishikori advanced to the third round of the U.S. Open for the second straight year on Wednesday.

Facing left-hander American Bradley Klahn for the first time, Nishikori emerged a 6-2, 4-6, 6-3, 7-5 winner from their match at Louis Armstrong Stadium. Nishikori wrapped up the match against No. 108-ranked Klahn in 2 hours, 44 minutes.

"When he was on a roll, he was serving very well. So it wasn't easy to return, but overall, I think I played good tennis," Nishikori said.

"Of course it's better to win in three sets in 1 or 2 hours, but today I learned a couple of things. Sometimes it's good (to get court time), especially in the first couple of rounds," Nishikori said.

After cruising to a one-set advantage, Nishikori went up 3-1 in the second set. But Klahn rallied to win the set's eighth game, and two games later crushed a forehand winner to pick up another break and level the match.

In the third set, Nishikori earned a key break chance with a brilliant forehand down the line, and capitalized on it with a backhand winner to go up 3-2. He grabbed one more break to secure the set and regain the lead.

Although the Shimane Prefecture native appeared to be running away with the fourth set at 5-1, he stumbled at a key point, committing game-ending double faults in both the seventh and ninth games that allowed his American opponent to draw even.

Following a love hold, Nishikori went up 0-40 against Klahn's serve for a look at triple match point. Klahn won the first two but sent a shot long to give Nishikori the match on the third.

The seventh-seeded Japanese will face either Alex de Minaur of Australia or Chile's Cristian Garin in the third round.

Four of the top 10 seeds in the men's draw, including world No. 4 Dominic Thiem of Austria, were knocked out in the opening round of the year's final Grand Slam.

The majority of Wednesday's second-round matches have been delayed due to persistent rains, affecting all courts except the venue's two covered stadiums.

Nishikori reached the final at New York's Billy Jean King National Tennis Center in 2014 and has reached the semifinals twice since.