Dimitrov overreaches and Nadal gets the first point, but makes it 15-15 with a hit at the net.
An 18-shot long crosscourt rally follows, with Nadal hitting one out, when he misses the angle on his backhand.
But he quickly makes up for it with a powerful smash that flummoxes Dimitrov, who hits another backhand at the net.
A wide shot makes it 4-3 in Nadal's favor after an unsuccessful review.
If Nadal wraps this up in 4, he'll have played *90* fewer minutes than vs. Verdasco in 2009, and Verdasco pushed him a LOT more than Grigor.
Nadal drives a backhand into the net to give Dimitrov the first point, but promptly follows wth his own shot at the net to level 15-15.
But Dimitrov induces a wayward shot to make it 30-15, before hitting his own outside for 30-30.
The Bulgarian then unleashes his big serve at an sharp angle and noses ahead and gets the game with another outside by Nadal.
3-3 in the fourt set
At 3-2 in fourth, Nadal-Dimitrov already 8 min longer than Federer-Wawrinka, a 3h05m five-setter with off-court medical timeouts.#AusOpen
Dimitrov drops one right on the line to take the first point thanks to his classic backhand winner, before Nadal evens it out.
Nadal's incisive serve is answered by a massive smash and an unforced error, levelling the game at 30-30.
A 21-shot long rally follows, where Nadal sneaks one past an close approaching Dimitrov to take the lead and a wayward shot gives the Spaniard a 3-2 lead in the fourth set.
Comfortable holds for both players as they even out the fourth set.
Dimitrov shows another glimpse of his classic backhand, as well as his unwitting unforced errors in the fourth game.
The Bulgarian will need more that trusty one-handed shot if he wants to outplay Nadal who looks to be giving no free points.
The Spaniard pulls the game to deuce with a timely smash, but Dimitrov comes through.
Preview: Rafael Nadal's path to his Australian Open semi-final against Grigor Dimitrov on Friday has taught the 30-year-old Spaniard one thing -- he's still got it.
After being bundled out in the first round by compatriot Fernando Verdasco last year, Nadal has reached his fifth semi-final at Melbourne Park in a remarkable turnaround for the 14-times grand slam winner.
A barren 2016 saw him fail to reach at least one grand slam final for the first time since 2004 and while he won Olympic doubles gold at the Rio Games a troubling wrist injury forced him to call time on his season in early October.
Rafael Nadal will play Grigor Dimitrov. AFP
Hard work has been central to the latter part of Nadal's career, particularly as knee and wrist injuries forced him to miss large chunks of recent seasons.
Questions were being raised as to whether he still had the physicality and game to match the likes of Novak Djokovic, Andy Murray, Stan Warwinka and the evergreen Roger Federer, as well as the new generation coming through.
The last three rounds at Melbourne Park have answered those questions decisively as Nadal beat 24th seed Alexander Zverev in five sets, sixth seed Gael Monfils in four, then outclassed third seed Milos Raonic in the quarter-finals.
Next up is Dimitrov.
Once tipped as the young player most likely to challenge the established order, the Bulgarian has struggled to live up to those expectations and was hindered by off-court distractions.
Friday will mark just his second grand slam semi-final but the 25-year-old feels that under new coach Daniel Vallverdu he now has the mental strength and attitude to match the game that prompted those expectations.
Things have indeed gone his way this year.
Dimitrov is on a winning run of 10 matches that earned him the Brisbane International title and has taken him to the last four at the Australian Open.
However, he knows that means little when he steps on court against Nadal, who has won seven of their eight previous encounters.
"In order to win a slam, there's no shortcut," the 15th seed said. "If you think about it, when have you seen an easy semi-final or something like that?
"You got to work for it. I have to fight, I have to be ready."