Westwood was rolling in his match against an exhausted Miguel Angel Jimenez until the Spaniard battled back, forcing their contest down to the wire.
'I am pleased with the way I played and I am proud of the way I played, especially this afternoon when I battled back and to take it to the last hole was a good effort,' said Jimenez. 'I knew if I could start hitting the ball a bit better I had a chance and that is what happened.'
Westwood took a 3-up advantage over Jimenez after the morning 18. Jimenez, who had to complete his quarterfinal match against Bernhard Langer early on Saturday, bogeyed the 19th hole to dig an even deeper hole and Westwood took a 5-up lead with a birdie at the 22nd hole, the par-5 fourth.
The Englishman looked to extend his lead even further after his approach to the 24th stopped within 8 feet of the hole. Jimenez had a long distance to cover on the green, but he holed his birdie try putting the pressure on Westwood.
Westwood made his putt to halve the hole, but the momentum was beginning to shift. Jimenez matched Westwood with a birdie at the 28th and went to 4-down after Westwood bogeyed the 29th.
Jimenez reached the green in two at the par-5 12th, the 30th hole, while Westwood left his second shot short of the putting surface and putted his third within a foot of the hole. Jimenez then drained his eagle try to close the gap to 3-down.
Jimenez won the following hole to go 2-down and spun his second shot back within a few inches of the cup for a birdie at the 34th hole. Westwood, who moments before had hit a solid approach to 7 feet, failed to convert and just like that his advantage was 1-up with daylight fading.
'When I hit it close at 16, I expected him to hit it close,' Westwood said. 'That's the way you play match play, don't take anything for granted and expect the worst.'
After delays forced the suspension of play on Thursday and Friday, Jimenez and Westwood were facing the prospect of having to return to the West Course Sunday morning to finish their match.
Jimenez was out of gas at that point, however, and Westwood held on for a 1-up win after he reached the par-5 closing hole in two.
'It boosts your confidence any time you win a game,' said Westwood. 'That's why these games are over 36 holes to see who is the best one over 36 holes.'
Els had a much easier time in his match against an ailing Harrington who injured his hand in the quarterfinals.
'I was really on early on and if it wasn't for those missed putts, it could have been a really good score,' said Els.
Els led 3-up after the morning session and increased his lead to 4-up after Harrington bogeyed the 21st hole. The duo halved the 22nd with birdies, but Harrington ran home a 20-foot putt for a birdie at the 23rd to get to 3-down.
'I think this morning I couldn't put my thumb on the club at all,' said Harrington. 'This morning I couldn't put it near the club, and after lunch I tried a few shots with it on the club and it wasn't too bad. Awkward, but not too bad.'
Els responded at the par-4 seventh, the 25th hole, and dropped his second inside 12 feet for a birdie to regain a 4-up lead. Harrington won the 26th with a birdie, but Els countered with a birdie at the 29th.
They halved the 30th with birdies and Harrington saw the match slip away after he bogeyed the 32nd.
'A lot of shots have been hit and played out there and I'm just delighted to get into the final,' said Els. 'It's one thing making the final, it's another thing trying to win.'
Els and Westwood are both premier players on the European Tour, but the South African has dominated this event over the last 10 years. While Els is in search of his sixth victory at this event, Westwood is no stranger to success in match play at Wentworth, having defeated Colin Montgomerie in a thrilling final to win the championship in 2000.
'I have a good record against Ernie here. I'm looking forward to tomorrow, which will hopefully be a great match for everybody to watch,' said Westwood. 'We'll have a good game.'