Third Test - South Africa v British and Irish Lions
Venue: Ellis Park, Johannesburg Date: Saturday 4 July Start: 1400 BST
Coverage: Live on Sky Sports, BBC Radio Wales, 5 Live and live text commentary on the BBC Sport website
Williams has three Lions caps after sitting on the bench seven times
Veteran flanker Martyn Williams says he was overjoyed after being selected for his first Test start on his third British and Irish Lions tour.
"It's the pinnacle for any player in Britain, I'm delighted," he said after displacing Ireland's David Wallace for the third Test against South Africa.
The 33-year-old Welsh open-side toured with the Lions in 2001 and 2005.
He has sat on the bench seven times, winning one cap in the third Test of 2005 and two more on this tour.
"Relief was the biggest emotion [after being selected]," Williams, who has 88 Wales caps and was a Grand Slam winner in 2005 and 2008, told BBC Sport.
"Knowing that this was the last time I would ever play for the Lions I was overjoyed when I heard that I would finally get that number seven jersey, it was a really proud moment.
"It was disappointing not to get a start before now, but if you look around the squad on a Lions tour it's always going to be tough.
"David Wallace has played outstandingly well throughout the tour and in the first two Tests.
"I'm told that it was a close call between us - we have great respect for each other."
Lions coach Ian McGeechan said: "It was nip and tuck between Martyn Williams and David Wallace, having fresh legs was important."
Pontypridd man Williams comes into a side that shows eight changes with the series already lost, South Africa having won the first two Tests.
"There's nothing worse than what-ifs in professional sport and that's what we've been feeling for the last two Sundays," said the Cardiff Blues flanker.
We feel that we've put a lot of pride back into the jersey after the disappointments of 2005
"We feel that we could have started the first Test better and closed out the game last week.
"But credit to South Africa, when it's been crunch time they've come up with the goods.
"We've had more time together now, though, so hopefully we can pull off a win this week."
Despite the disappointments of the tour, Williams says he has nothing but gratitude to Wales and Lions coach Warren Gatland for coaxing him out of a short-lived international retirement after the 2007 World Cup.
"I still owe [him] a huge amount for that," said Williams.
"I'm going to be playing at Ellis Park in front of a full house, it's really exciting.
"It's been an 'old-school' tour where we've all mixed really well and we'll have a lot of great memories - I'm sure that the Lions will go from strength to strength in years to come.
"[The Lions squad] feel that we've put a lot of pride back into the jersey after the disappointments of 2005 and we've spoken about how it will be if we don't front up on Saturday.
"There's definitely plenty of motivation with a lot of us making our first Lions starts.
"It's a huge Test against South Africa and we want to go home with a win under our belts."