NEW HAVEN, Conn. – Juan Martin del Potro, who has won four straight times on the ATP Tour, withdrew from the Pilot Pen tournament Monday.

No immediate reason was given for the withdrawal, but a news conference was planned for later in the day.

Del Potro, ranked 17th in the world and seeded third at New Haven, was coming off tournament victories in Stuttgart, Kitzbuhel, Los Angeles and Washington, where he won the championship match Sunday. He had been scheduled to play Tuesday.

Lucky loser Jesse Levine will take his place in the main draw.

On the court Monday, promising young American player John Isner defeated Igor Kunitsyn 6-4, 6-4 to advance to the second round.

The hard-serving Isner closed out the match with a 135 mph serve, but said the key to victory was throwing in some changeups.

“I tried a different strategy,” Isner said. “I didn't go for the big bomb every time. I thought I'd try some different things heading into the (U.S.) Open.”

This was the first time Kunitsyn faced the 23-year-old Isner in singles.

“It was an extremely bad first-round draw coming late into the tournament,” said Kunitsyn, who had just come from Washington, where he advanced to the semifinals. “This is the last guy you want to play to get your rhythm.”

The 6-foot-9 Isner next faces 6-10 second seed Ivo Karlovic in what will be their first meeting.

“That will be interesting for sure,” said Isner, who made his pro debut last year. “If I somehow play well enough to win, that will be huge.”

Fellow American Robby Ginepri also advanced to the second round after defeating Luis Horna 7-6 (4), 6-2.

Top seed Fernando Verdasko was scheduled to play Monday night.

Fifth-seeded Patty Schnyder defeated Jill Craybus 6-4, 7-6 (2), extending her record against the American to 4-0 in a first-round match.

Schnyder, ranked No. 15 in the world, also beat Craybus in the first round of the Beijing Olympics, 6-3, 6-2. Schnyder was eliminated in Beijing by Sybille Bammer in the second round and said she still felt the effects Monday of the 18-hour flight from China to Connecticut.

“Generally it is tough to come from Beijing,” Schnyder said. “It is tough on your legs. It is just your body needs more time.”

The 85-degree heat Monday didn't help either.

“I felt really weak in the second set,” Schnyder said. “Luckily there was a little breeze that cooled me down. It was tough conditions.”

Eighth-seeded Dominika Cibulkova retired from her match because of heat illness after losing the first set to Caroline Wozniacki 7-6 (7).