2002-2004

Lapentti announced his ATP debut in 2002, but he actually had his first tour match in 2003, at the Franklin Templeton Classic in Scottsdale, Arizona, United States. Lapentti obtained his first professional victory at that tournament's first round, defeating Paradorn Srichaphan, 7–6 (7–2), 6–2. He lost in the tournament's second round to David Sánchez.

He participated in 2003's Italian and French Open, losing in the first round both times. During the French Open, he was leading 2 sets to 1 against Tommy Robredo but had to retire due to an injury while trailing the fourth set; the injury did not allow him to return to the professional men's tennis tour until 2004.

Lapentti did not obtain a single victory during his second year as a professional, but he participated in tournaments held in Argentina, Mexico, Spain and Indian Wells, California.

2005-2013

Lapentti had better results in 2005, losing in the Davis Cup playoffs to Jürgen Melzer (he only played one Davis Cup game that year), advancing to the second round of the Legg Mason Classic, where he was defeated by Andy Roddick, getting to an ATP's tournament's third round for the first time in his career when he participated at the RCA Championships (losing to Paul Goldstein in the third round), making the second round of the Campbell's Hall of Fame Championship and the third round of the Tennis Channel Open. The only tournament that he participated in and failed to reach the second round during 2005 was the Pacific Life Open in Indian Wells, where he lost to Kevin Kim.

Lapentti was eliminated in the first round of 2006's Pacific Life Open, that time by Mardy Fish. In June 2008 Lapentii won the Manta Open and had accomplished his goal also winning Costa Rica Seguros Bolivar Open in March 2011.