It remains within the realms of possibility, if only just, that this Sunday's ATP Tour Championship final could be between Tim Henman and Greg Rusedski.

Fantasy tennis, perhaps, but this has been a decidedly strange week, made the more so yesterday when Chile's Marcelo Rios followed in the footsteps of Andre Agassi and quit the tournament, thereby handing Pete Sampras his cherished ambition of being the world number one for a record sixth successive year.

"It's a phenomenal achievement, and I don't think it's going to be broken for a long, long time - if ever," said Henman, who had just made his own bit of history by becoming the first British

player to reach the semi-finals of this event after a tense 7-6, 6-7, 6-2 victory over Spain's Alex Corretja.

"It was an extremely tough match with lots of changes of momentum," said a perceptibly relieved Henman who, immediately after leaving court, had to double-check that he was indeed in the last four.

The round-robin format of the first four days has had everybody on edge. In fact, Henman had not only qualified, but also clinched his place at the top of the White group, thereby avoiding Sampras in the semis.

His opponent tomorrow will be either another Spaniard, Carlos Moya, or his old protagonist Yevgeny Kafelnikov of Russia. But prior to that there is the little matter of his final round-robin match against Rusedski.

Henman's eyes glittered bright at the prospect. "I don't like losing any matches but I think we'll be extra keen to beat each other."

Rusedski, the first reserve, woke up yesterday expecting to meet Rios but ended up, at under five minutes' notice, playing against the second reserve, Albert Costa of Spain, who arrived here with the unenviable record of never having won an indoor match. He duly maintained this form by losing 7-6, 6-1 to Rusedski, who admitted to having an bad attack of nerves in the opening set.

For Rusedski to join Henman in tomorrow's semi-finals, and thereby play Sampras, he must both beat his fellow Brit and then wait to see if Costa can defeat Corretja. "Do you expect the two Spaniards to play a friendly match?" asked an Italian journalist. The gist of Rusedski's reply was "no way".

Obviously, the chance of an all-British final is very remote, although the possibility of Henman meeting Sampras, who won his final round-robin match against

Slovakia's Karol Kucera 6-2, 6-1, is altogether more probable. The two have already met twice this year, in the semi-finals at Wimbledon and indoors in Vienna, Sampras winning both.

"I've beaten a lot of top players this year. If it doesn't happen that I beat Pete it's definitely going to be a goal next year," said Henman.

Having beaten a decidedly under-par Rios in his opening match, Henman's encounter with Corretja was always likely to be a much sterner examination of his form and nerve.

These days the Spaniard laughs if he is described as a clay-court specialist. And quite right so. "He can compete on any surface, and hits the ball very heavy," said Henman.

The opening was immensely intense. There was no suggestion of a break point on either serve, and a tie-break became inevitable. By now Henman's tactic of rushing to the net was beginning to force Corretja into ever more risky shots, and one backhand hit wide of Henman but also wide of the line was enough for the British number one to secure the first set. It was that close.

When a set is so tight, it is often followed by the complete opposite, and so it was, with both men having their service broken twice in the opening five games. Yet at 6-3, 30-love, Henman was two points away from a straight sets victory before a couple of lousy service returns on both wings allowed Corretja back in. This time Corretja won the tie break and, with his renowned stamina, appeared to have taken the high ground. But having drawn level Corretja fatally relaxed, and Henman was on him like a hawk.

Henman and Rusedski have met twice on the ATP Tour, Henman winning in Ostrava two years ago, and Rusedski beating him in Vienna last year. "It's great for British tennis and if I beat Tim I would finish in the top 10 again." said Rusedski.