Cristiano Ronaldo's astounding early goal left Sir Alex Ferguson to go head-to-head with old rival Arsene Wenger for a place in the final.

The emergence of Chelsea and Liverpool may have taken the edge off battles between Manchester United and Arsenal over the past few years.

But the renewal of hostilities promises to be almost as spectacular as the Ronaldo strike that created it.

The sixth-minute effort was his 20th goal of the season and only his second in the Champions League since completing that astonishing haul of 42 in last year's final in Moscow.

There was certainly no doubting it was worthy of winning any game, and earning United the accolade of becoming the first English team to beat Porto on its own ground.

As almost a side issue, it also keeps the Red Devils on track to win the quintuple in a season they have brought back to life at just the right time.

In calling for the spirit of Turin '99, Ferguson also knew if that momentous evening was to be repeated, he would have to go through the agony of being two goals down before a famous victory could be achieved.

Ronaldo, however, has his own history book to fill.

A repeat of last season's 42-goal tally was never likely to be repeated in a campaign delayed because of ankle surgery.

But the 24-year-old has still make a significant contribution to United's attempt to land an unprecedented quintuple, even if, like so many of his team-mates, he has looked a little off the pace in recent weeks.

There could have been no better stage to signal his renewed intent.

Apart from his beloved Sporting Lisbon, Ronaldo is roundly jeered when he returns to one of the major clubs in his homeland, Porto being no exception.

They recognise his talent, though, even if no-one expected what followed when Ronaldo collected Anderson's short pass closer to the halfway line than the opposition goal.

Sensing a little trepidation in home ranks, Ronaldo let rip with a shot as brutally powerful as it was audacious in its intent.

Porto keeper Helton did not have a chance of keeping the ball out as it flashed past him to give United the start Ferguson could not have dreamed.

Had Nemanja Vidic been able to keep his close-range effort under the crossbar after John O'Shea had got the faintest of touches to a Ryan Giggs corner just before the interval, the Red Devils would surely have completed their task in half the time allotted.

Porto mount numerous offensives in the second half but with Rio Ferdinand back alongside Vidic after his recent groin strain there was an authority about United's defending that had not been in evidence since before it was dismantled by Liverpool five games ago.

For Arsenal, which has been portrayed as a club in crisis this season, it is now in the Champions League semi-final after sweeping past Villarreal 3-0 at the Emirates Stadium.

Theo Walcott, Emmanuel Adebayor and Robin van Persie were on the scoresheet as Arsene Wenger's youngsters defied their injury worries in defence to wrap up the tie within an hour.

The Spaniards had Sebastian Eguren dismissed, and the hosts were then able to conserve energy for their FA Cup clash against Chelsea at the weekend.

If they win at Wembley there will be genuine hope of getting something out a season where many predicted them to slip out of the traditional top four in England.

The 20-year-old Walcott showed his ruthless side again when he netted the opener in the 10th minute.

Emmanuel Eboue slipped the ball forward from the right, Cesc Fabregas' flick allowed Walcott to run through on goal, with the England winger chipping home high over goalkeeper Diego Lopez.

It was reward for Arsenal's bright start after Van Persie went close with a header that cleared the crossbar and a long-range effort that Lopez gathered at the second attempt.

The hosts were inches from adding a second goal after Van Persie's powerful free-kick was saved by Lopez. Adebayor's follow-up header got deflected on target but Gonzalo Rodriguez hacked off the line.

All eyes were on the assistant referee but it would have been the tightest of calls if he had flagged for a goal.

The equation for Villarreal shifted only slightly - it still needed a goal.

Diego Godin had given Wenger's side an early scare when he got on the end of Giuseppe Rossi's free-kick and forced Fabianski into a save with his scissor-kick.

No opponent got close enough to Van Persie on the hour mark and he sent through Adebayor, who took one touch before finishing past Lopez with the outside of his boot.

The third came from Van Persie's penalty and also saw Eguran dismissed.

Godin took Walcott's legs away in the penalty area but escaped a second booking for the offence. Eguran, who got a yellow card in the first half, got another caution for dissent and then disgracefully put his hands on the referee while protesting more. Van Persie kept his nerve and fired in the spot-kick.

Wenger's goalscorers were then given a breather to rest for the battles ahead.

Arsenal 3 (Theo Walcott 10, Emmanuel Adebayor 60, Robin van Persie 69 pen)
Villarreal 0
Arsenal wins 4-1 on aggregate

Porto 0
Manchester United 1 (Cristiano Ronaldo 6)
Manchester United wins 3-2 on aggregate