John Part
Former PDC world champion on Sky Sports

Josh Rock can't allow Chris Dobey to win this leg with straight tons. He's got to put pressure on.

Chris Dobey was on for a 10-darter but misses three at double eight.

That gives Josh Rock a chance to steal after a 180 on his previous visit.

But he's unable to give himself a shot at the outer ring and Dobey comes back to hit double four to seize control of this set.

What's this? A hold to start a set? Who'd have thought it?

Chris Dobey overcomes a pair of misses on double nine to secure the hold on double four.

That's four legs on the spin for Dobey who begins the next with a 180...

John Part
Former PDC world champion on Sky Sports

These players have performed as we expected.

Chris Dobey has been the steadier of the two, particularly with his scoring.

Josh Rock has had the more explosive game and that works in the set format so that is OK.

The averages have jumped up since the first set.

Chris Dobey is now at 95.48, while Josh Rock is averaging 93.94.

But while Dobey has upped his checkout percentage to 40, Rock is still hovering around 30%.

Chris Dobey looks to be out of the leg on 186 with Josh Rock needing 60.

But the Englishman goes for the 180 despite it leaving the tricky double three.

That causes something of a distraction for Rock, who misses two darts for tops, then another three doubles when Dobey is unable to get the double three.

Dobey gets it at the fourth opportunity, though, and we're all square again.

What a game to start the session!

Chris Dobey consolidates the break with an 11-dart hold.

He finishes it off by taking out 96 in two darts.

Every time one player looks to have the momentum, the other responds and now we're a leg away from Dobey drawing level again.

You know the drill by now in this game... every set starts with a break of throw.

Chris Dobey is the beneficiary on this occasion. Nailing double four for a 15-darter.

John Part
Former PDC world champion on Sky Sports

Josh Rock is doing things as he would like at the moment but he knows he is only halfway to the finish line - there is a long way to go yet.

Image source, Getty Images

Josh Rock was flying but threatened to let Chris Dobey back in.

Not for long, though, as he gets himself going again with a superb 177.

From there he hits a 132 to leave tops and, when Dobey fails to take out 106, he comes back to hit it first time.

Rock leads two sets to one.

Josh Rock misses two darts for the set allowing Chris Dobey a second look at double 16.

It's a double that has served him well in this game and does so again here.

Dobey holds to extend the set.

Superb leg from Josh Rock - an 11-darter.

There were boos from the crowd when he couldn't follow up his 180 with another to set-up a potential nine-darter.

But that turned to cheers when he got another 180 on his next visit. Double 16 finishes the job to move him to within a leg of taking the third set.

Three sets and we've had a break of throw to start them all!Josh Rock takes it with a 68 finish.

And he starts the next leg with a 180...

John Part
Former PDC world champion on Sky Sports

Chris Dobey was really disgruntled in the first set but he answered really well in the second set. He looked quite serious.

Image source, PA Media

Darts is one of the few sports that can measure perfection; snooker has the 147 maximum break, golf has the hole-in-one, darts has the nine-dart finish.

The fewest number of darts required to finish a standard leg of 501 points is nine.

There are several ways to complete a nine-dart finish, the most common being two visits of 180 followed by a 141 checkout.

The most recent nine-darter at the PDC World Championship was earlier on Friday by Australia's Damon Heta against Luke Woodhouse. He went on to lose, though.

It was the second of this year's tournament after Christian Kist's (pictured above) on 17 December.

There is a method of completing a nine-darter with three identical visits of 167 - treble 20, treble 19, bull - although that is usually saved for the exhibition circuit rather than in tournament play!

Image source, Getty Images

Chris Dobey hits double 16 again to make it all square after two sets.

An average of 108 in the set from the Englishman.

Things really started to heat up in that second set. This could be an absolute cracker.

Dan Dawson
Commentator on Sky Sports

It has taken a little while for the machines to get going but they are purring now.

John Part
Former PDC world champion on Sky Sports

Even in missing that 170 it was outrageous from Josh Rock!

What a leg!

It's raining 180s at Ally Pally! Josh Rosh gets it started but Chris Dobey outdoes him by going back-to-back.

Rock then almost reels in the big fish, hitting the two treble 20s but finds the 25 rather than the bullseye.

That leaves Dobey to take out 45 in two darts to break again. The first hold of the set would level the match for Dobey.

Image source, Getty Images

Josh Rock breaks straight back.

The second set is mirroring the first so far.

A 145 set-up leaves the Northern Irishman needing 41 and he gets it done, with Chris Dobey waiting to pounce, with a double eight.