Novak Djokovic took another important step to defend his U.S. Open title Thursday night, defeating Juan Martin del Potro 6-2, 7-6 (3), 6-4 to advance to the semifinals.

Djokovic took advantage of notoriously slow-starter del Potro in the first set, breaking the seventh-seed to take a 3-1 advantage. The two played an exciting 31-shot rally that ended on a del Potro shot to the net that was a sign of things to come later in the match.

"We played some incredible rallies and incredible points,'' Djokovic told the Associated Press. "It's always entertaining, always so much fun, playing in these night sessions.''

After fighting off two set points, Djokovic broke through on a perfect drop shot to take the first set on del Potro's serve. Coming into the match, Djokovic had averaged two breaks of serve per set, the highest average of any player in the tournament.

The second set was a drama filled affair and was the turning point in the match.

The second-seeded Djokovic was broken in the first game at love, losing on a double fault that put del Potro up 1-0. He continued to dominate, winning ten straight points to start the set, but Djokovic held his next serve to cut it to 2-1 on a fierce forehand winner.

The Argentine went up 3-1 after a game that featured a dramatic 27-shot rally that he won on a fantastic backhand return.

After trading service games back and forth, Djokovic finally broke back, evening up the set at 5-5. He won the next game to take a 6-5 advantage before the two dueled in what would be the game of the match.

The nearly 20-minute frame had del Potro serving to force a tiebreaker. The two battled through six deuces and multiple set points, including a cross court winner by del Potro that saved the set and put him up an advantage. After a long return on a hard serve by Djokovic, del Potro forced the tiebreak.

Commentator John Mcenroe probably described the lengthy second set game best, saying "set point it might as well be match point."

Del Potro took an early lead in the tiebreak but Djokovic fought hard, slamming multiple winners to take a 5-3 lead. The next point was the most enthralling of the match.

The 20-stroke point featured a range of near-impossible returns, including a lob shot from del Potro that forced Djokovic on the run. After an overhead slam, del Potro somehow recovered to keep the ball in play, after which another drop shot forced the Argentine to sprint to the net. He pushed a bit too hard and the ball went long.

An exasperated del Potro leaned down on the net after the point, while Djokovic pumped towards the crowd. He closed out the set on the next point.

"Crucial," Djokovic said about Thursday's second set. "It could have gone either way."

The second set lasted for nearly 90-minutes and gave Djokovic a commanding 2-0 lead.

"I had the chance to win that set," del Potro said to the Associated Press, "but Nole played really well, basically, in the important moments."

Djokovic broke serve in the next set and won 6-4 to advance to the next round.

Del Potro and Djokovic faced off over the summer at the London Olympics, with del Potro emerging victorious and earning a bronze medal.

This is the 10th consecutive Grand Slam semifinal that Djokovic has reached and the sixth-straight at the U.S. Open. He will play the fourth-seed, David Ferrer on Saturday. Ferrer had a wild four hour match against Janko Tipsaravec, winning 6-3, 6-7 (5), 2-6, 6-3, 7-6 (4) on Thursday.