The 18-year-old Harrison, based in Bradenton, Fla., qualified for the U.S. Open. He made his Grand Slam debut in January at the Australian Open, losing in the first round.
He's considered one of the top young U.S. players.
Ljubicic was a French Open semifinalist in 2006 and reached a career-best ranking of No. 3 that year. But he's never been past the third round at Flushing Meadows and also lost in the first round in 2009.
Clijsters advances to third round of US Open
Defending champion Kim Clijsters advanced to the third round of the U.S. Open with a 6-2, 6-1 victory over Sally Peers.
The second-seeded Clijsters needed only 56 minutes Wednesday night in Arthur Ashe Stadium to defeat her 19-year-old opponent, an Australian qualifier whose previous encounter with the Belgian star came when she got her picture taken with Clijsters several years ago.
Clijsters, who came out of retirement to win the U.S. Open last year, is trying to become the first woman to successfully defend her title at Flushing Meadows since Venus Williams in 2001.
Clijsters plays 27th-seeded Petra Kvitova in the third round.
Roddick upset by Tipsarevic in 2nd round of Open
Andy Roddick was upset in the second round of the U.S. Open, losing 3-6, 7-5, 6-3, 7-6 (4) to 44th-ranked Janko Tipsarevic of Serbia on Wednesday night.
The ninth-seeded Roddick won his only Grand Slam title at Flushing Meadows in 2003, and he also was the runner-up in 2006. His loss leaves Roger Federer as the only past men's champion in the field.
This early exit follows a fourth-round loss for Roddick at Wimbledon. He revealed recently that he had a mild case of mononucleosis.
Tipsarevic moves into the third round at the U.S. Open for the first time.