He turned professional in 2000. In 2001, he finished in the ATP top 50 for the first time. He finished 2002 as No. 1 Argentine and South American for the first time in his career, winning two ATP titles and reaching the Wimbledon final, where he lost to Lleyton Hewitt. His run to the 2002 Wimbledon final is remarkable, considering that it was his first ever pro tournament on grass.
Nalbandian wasn't able to return to the Wimbledon Championship in 2003, as he was knocked out in the Round of 16 to hometown favorite Tim Henman. He did however make an impressive run at the U.S. Open, taking out both of the Wimbledon finalists Mark Philippoussis, and Roger Federer along the way to a semifinal matchup with Andy Roddick, it was the second time in less then a month that he would meet Roddick in high stakes match, having lost to him in the final of Rogers Cup a few weeks earlier, Nalbandian took a two sets to love lead, and held match points in the third, there were several controversial calls that didn't go his way, he would eventually lose the match 6–7, 3–6, 7–6, 6–1, 6–3, Roddick went on to win the championships. He finished 2003 ranked No.8 in the world.
In 2004, Nalbandian made his best effort at the French Open reaching the semifinals, again he lost to the eventual champion Gastón Gaudio. He finished 2004 ranked No. 9 in the world, despite breaking into the top 5 for the first time in his career in July.
In 2005, Nalbandian advanced to the quarterfinals of the Australian Open, Wimbledon, and the U.S. Open, more importantly he won the Tennis Masters Cup, becoming only the second Argentine tennis player in history (after Guillermo Vilas in 1974) to win the end of year tournament. Having replaced American Andy Roddick, Nalbandian won two of his three group matches (l. to Roger Federer, d. Ivan Ljubičić and Guillermo Coria). In the semi-finals he defeated Russian Nikolay Davydenko and in the final, he beat World No. 1 Roger Federer in the fifth-set tiebreak. David also becomes the first player to win the cup without previously attaining a Grand Slam or Masters Series title.
Nalbandian then competed in the Davis Cup Semi Final against Australia. David easily defeated Mark Philippoussis 6–4 6–3 6–3 to give Argentina a 1–0 lead. Nalbandian felt as if the crowd didn't get totally fired up because he was cruising through the match. When the draw for the Davis Cup semifinal was being made with Argentina vs. Australia, Nalbandian shook hands with Australian Mark Philippoussis and asked him to join the party on Saturday night, implying that Argentina would beat Australia 3–0 on Saturday. Nalbandian was indeed correct, and Argentina went on to thrash Australia 5–0. Nalbandian said the only thing he was disappointed that he didn't have the opportunity to beat Lleyton Hewitt, whom he thrashed in the Davis Cup quarterfinal in 2005 6–2 6–4 6–4 in Australia (Nalbandian's record against Hewitt is 1–3). Prior to the Davis Cup semifinal, Nalbandian said, "We beat them in Australia, and we're going to beat them again." Argentina went on to humiliate Australia, thrashing them 5–0. Nalbandian also fired up the clash with Australia by saying that Hewitt has no friends on the ATP Tour.
2007 has seen Nalbandian drop out of the world's top twenty for the first time since 2003 after losing in the fourth round of the 2007 French Open to Nikolay Davydenko. Nalbandian suffered various abdominal injuries, a back injury and a leg injury during the year which is the reason for his form lapse.
He fell to No. 26 in the world until his season changed when after winning the 2007 Madrid Masters. He won the tournament by defeating Rafael Nadal, Novak Đjoković, and Federer in consecutive rounds, becoming the third player after Boris Becker and Đjoković to defeat the world's top three players in a single tournament. He defeated Nadal, 6–1, 6–2, in the quarterfinals. His good form carried him to his second top three win of the tournament, defeating Djoković in the semifinals. He then stunned Federer in the final, winning 1–6, 6–3, 6–3.
He also reached the doubles semifinals with Guillermo Canas in the tournament before losing to top-seeded Bob Bryan and Mike Bryan in the semifinals. After the tournament, his singles ranking moved up from No. 25 on the tour to No. 18, losing in the first round of his next tournament.
Nalbandian played at the 2007 Paris Masters and again defeated Federer in the third round 6–4, 7–6 (7–3). He then beat David Ferrer in the quarterfinals in a closely fought match, winning 7–6 (7–3), 6–7 (3–7), 6–2. After beating Richard Gasquet 6–2, 6–4 in the semifinals, Nalbandian won his second straight ATP Masters Series title over Nadal, 6–4 6–0, thus, becoming the first player to win Madrid and Paris Masters back-to-back since former World No. 1 Safin in 2004. This win allowed Nalbandian to move back into the top ten at No. 9.
Nalbandian began 2008 back in the top ten, however for the seventh straight time he failed to make the Quarterfinals of a slam, suffering a 6–1, 6–2, 6–3, 3rd round loss to the No. 22 seed Juan Carlos Ferrero. On February 24,2008 Nalbandian was able to win the Copa Telmex on his own home soil in Buenos Aires. He beat fellow compatriot Jose Acasuso 3–6 7–6(7–5) 6–4. With that win he was able to move to the number 8 ATP rank.
The following week, Nalbandian arrived at the Abierto Mexicano Telcel in Acapulco, Mexico. Nalbandian defeated Boris Pašanski, Santiago Ventura, Potito Starace, and Luis Horna all in straight sets, but then lost in the final to Nicolás Almagro, 6–1, 7–6 (1).
Nalbandian entered the 2008 Pacific Life Open, and received a "bye" in the first round because of his seventh seeding. In the second round, he defeated Ernests Gulbis 6–4, 4–6, 7–6 (4), and then came back to beat Radek Štěpánek, 7–6 (1), 0–6, 7–6 (6). In the fourth round, he avenged Juan Carlos Ferrero for his Australian Open loss 6–2 6–2. He lost in the quarterfinals against Mardy Fish, 3–6, 7–6 (5), 6–7 (4), in a very tough match. He teamed up with Guillermo Cañas in doubles, but lost a close match in the second round. He then lost at the 2008 Monte Carlo Masters to eventual finalist Roger Federer 5-7, 6-2, 6-2. In Barcelona, he was the third seed, but was eliminated by Stanislas Wawrinka in the third round. At the 2008 Rome Masters, he fell in his debut to Nicolás Almagro 6-4, 7-5.
At the French Open, Nalbandian suffered a shock loss in the second round to Frenchman Jeremy Chardy. After being two sets up and seemingly in control, he lost the next three to hand Chardy the win; 3-6 4-6 6-2 6-1 6-2. After the French Open Nalbandian made his debut at the grass courts of Queen's Club Championships in order to prepare for Wimbledon. He reached the Semi Finals where he lost heavily to world number three Novak Djokovic 6-1 6-0.
At Wimbledon, Nalbandian suffered an early exit for the second straight Grand Slam tournament, going down to wild card Frank Dancevic in straight sets. 
In the US Open, Nalbandian lost in the third round in straight sets against Frenchman Gael Monfils.
| Year | Championship | Opponent in Final | Score in Final |
| 2002 | Wimbledon | Lleyton Hewitt | 6–1, 6–3, 6–2 |
| Year | Championship | Opponent in Final | Score in Final |
| 2005 | Shanghai | Roger Federer | 6–7(4), 6–7(11), 6–2, 6-1, 7–6(3) |
| Year | Championship | Opponent in Final | Score in Final |
| 2007 | Madrid | Roger Federer | 1–6, 6–3, 6–3 |
| 2007 | Paris | Rafael Nadal | 6–4, 6–0 |
| Year | Championship | Opponent in Final | Score in Final |
| 2003 | Montreal | Andy Roddick | 6–1, 6–3 |
| 2004 | Rome | Carlos Moyà | 6–3, 6–3, 6–1 |
| 2004 | Madrid | Marat Safin | 6–2, 6–4, 6–3 |
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| No. | Date | Tournament | Surface | Opponent in the final | '''Score |
| 1. | April 8, 2002 | Estoril, Portugal | Clay | Jarkko Nieminen | 6–4, 7–6(5) |
| 2. | October 21, 2002 | Basel, Switzerland | Carpet (i) | Fernando González | 6–4, 6–3, 6–2 |
| 3. | May 1, 2005 | Munich, Germany | Clay | Andrei Pavel | 6–4, 6–1 |
| 4. | November 20, 2005 | Tennis Masters Cup, Shanghai, China | Hard (i) | Roger Federer | 6–7(4), 6–7(11), 6–2, 7-5, 7–6(3) |
| 5. | May 7, 2006 | Estoril, Portugal | Clay | Nikolay Davydenko | 6–3, 6–4 |
| 6. | October 21, 2007 | Madrid, Spain | Hard (i) | Roger Federer | 1–6, 6–3, 6–3 |
| 7. | November 4, 2007 | Paris, France | Hard (i) | Rafael Nadal | 6–4, 6–0 |
| 8. | February 24, 2008 | Buenos Aires, Argentina | Clay | José Acasuso | 3–6, 7–6(5), 6–4 |
| No. | Date | Tournament | Surface | Opponent in the final | Score |
| 1. | October 1, 2001 | Palermo, Italy | Clay | Félix Mantilla | 7–6(2), 6–4 |
| 2. | July 8, 2002 | Wimbledon, London, England | Grass | Lleyton Hewitt | 6–1, 6–3, 6–2 |
| 3. | August 11, 2003 | Montreal, Canada | Hard | Andy Roddick | 6–1, 6–3 |
| 4. | October 27, 2003 | Basel, Switzerland | Carpet (i) | Guillermo Coria | W/O |
| 5. | May 10, 2004 | Rome, Italy | Clay | Carlos Moyà | 6–3, 6–3, 6–1 |
| 6. | October 25, 2004 | Madrid, Spain | Hard (i) | Marat Safin | 6–2, 6–4, 6–3 |
| 7. | November 1, 2004 | Basel, Switzerland | Carpet (i) | Jiří Novák | 5–7, 6–3, 6–4, 1–6, 6–2 |
| 8. | March 1, 2008 | Acapulco, Mexico | Clay | Nicolás Almagro | 6–1, 7–6(1) |
| Tournament | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | Career win-loss |
|---|---|---|---|---|---|---|---|---|---|---|
| Australian Open | A | A | 2R | QF | QF | QF | SF | 4R | 3R | 21-7 |
| French Open | A | LQ | 3R | 2R | SF | 4R | SF | 4R | 2R | 20-7 |
| Wimbledon | A | A | F | 4R | A | QF | 3R | 3R | 1R | 17-6 |
| U.S. Open | A | 3R | 1R | SF | 2R | QF | 2R | 3R | 3R | 17-8 |
| Grand Slam Win-Loss1 | 0-0 | 2-1 | 9-4 | 13-4 | 10-3 | 15-4 | 13-4 | 10-4 | 5-4 | 77-28 |
| Indian Wells Masters | A | A | 2R | 1R | A | 4R | 4R | 4R | QF | 10-6 |
| Miami Masters | 1R | 1R | 1R | 3R | A | 3R | SF | 3R | 2R | 7–8 |
| Monte Carlo Masters | A | A | 3R | 2R | QF | A | 3R | 2R | QF | 11–6 |
| Rome Masters | A | A | 2R | 1R | F | 1R | SF | A | 2R | 10-6 |
| Hamburg Masters | A | A | 1R | SF | 1R | 1R | A | A | A | 4–4 |
| Canada Masters | A | A | QF | F | 1R | 2R | 1R | 3R | A | 11-6 |
| Cincinnati Masters | A | A | 1R | QF | A | 2R | 2R | 1R | A | 5–5 |
| Madrid Masters | A | A | 3R | A | F | SF | SF | W | 18-4 | |
| Paris Masters | A | A | 2R | A | A | 2R | A | W | 7–2 | |
| Tennis Masters Cup | A | A | A | RR | A | W | SF | A | 6–6 | |
| Total Titles | 0 | 0 | 2 | 0 | 0 | 2 | 1 | 2 | 7 | |
| Year End Ranking | 245 | 47 | 12 | 8 | 9 | 6 | 8 | 9 | N/A |