The 36th-ranked Mertens is the first Belgian woman to reach the last four of a major since Kirsten Flipkens did it at Wimbledon in 2013 and the first to do it in Melbourne since Kim Clijsters in 2012. She'll face second seed Caroline Wozniacki for a spot in the final.
Mertens gets off to a fast start
With such a big opportunity in front of them to reach a Slam semifinal, both players were shaky on serve to begin the match, each fighting off break points in their opening service games.
It was Mertens who set up 0-40 on Svitolina's serve with some powerful winners off of the ground and she duly took it for a 2-1 lead, drilling a backhand crosscourt past a helpless Svitolina.
Both players settled down on serve, but it was the world number 36 who was on the offensive, dictating play from the baseline and approaching the net at just the right time.
With a chance to all but put the set away, the Belgian hammered another backhand down the line on break point for a 5-2 lead, seemingly hitting winners with ease.
Mertens' first and only blip in the match came next as Svitolina broke and then held to reduce the deficit to 5-4, but given a second chance to serve for the set, the Belgian made no mistake, holding to 30 to grab the lead.
Mertens serves up bagel to reach first Grand Slam semifinal
The aggressive nature that Mertens showed carried over to the start of the second set, holding a 0-40 lead only to see Svitolina peg her back. Given a fourth chance, she again aggressively attacked, finishing with an overhead smash to grab the break.
After a quick hold, the Belgian was on the attack again and after overturning a 40-15 lead on the Ukranian's serve, she got a slice of good fortune from the net on break point, Svitolina's backhand subsequently finding the tape, handing Mertens a 3-0 double break lead.
Falling behind 0-30 on her next service game, the world number 36 battled back and held for a 4-0 lead. Sensing her opponent's suddenly crumbling game, leaking errors from everywhere, the Belgian proceeded to break again to give herself a chance to serve out a bagel set.
Svitolina, knowing the end was just about near, didn't give up easily and she made the Belgian work for it, pushing Mertens to 30-30. Aggressively charging forward for a rare time in the match, the Ukranian duffed an easy swinging volley to bring up match point.
Capping what was a spectacular performance, Mertens blasted one final backhand winner to reach the final four. Following the match, she said “I have no words; I don’t know what to say. I have so many mixed emotions. I gave it all today, I fought. I got a little stress at the end so I played, my game and it went well today.
“She’s a great mover, she’s a really strong player. She started well at the beginning of the year so I knew it was going to be tough. So I tried to play aggressive, make her move, come to the net a bit more – just tried to play aggressive on the court.”
There's nothing Mertens can't do on a tennis court and she showed it today. Dictating play throughout, the Belgian hit 26 winners, showed her skills at net, winning 15/20 points when coming forward, possesses a terrific serve, winning 66% of her first serves and opportunistic in her chances, converting five of nine break points. Add in that she is a fine mover, tall and athletic and the result should not come as much of a surprise to those who saw it. If she plays to this level the remainder of the tournament, she will be holding the trophy come Saturday.
Svitolina was far too passive in the big moments, often a criticism of hers, only trying to up the pace and apply the pressure sporadically and when it was far too late. She's made significant improvements in her physical game, but her mentality must shift in order to take that next step to become a major champion. It's likely the Ukranian will get there, but until she brings her best in a major quarterfinal to reach the final weekend (she's now 0-3 lifetime in that round), she remains a question mark at this point.