Serena Williams admitted her straight sets quarter-final victory over sister Venus at the US Open felt every bit as tense as a final.
The 26-year-old trailed 5-3 in both sets and faced set points on both occasions but battled back to come through 7-6 (8-6) 7-6 (9-7) and secure a place in the semi-finals for the first time in six years.
Serena triumphed here in 1999 and 2002 and if she repeats that success this year she will become the world number one for the first time since a 57-week reign from 2002-03.
"It definitely felt like a final of the US Open," Serena said. "I can't believe I won."
"It was so hard. It is difficult mostly because she's such a great player. She was serving balls 125, 127 miles an hour. I just tried to stay with her."
Serena took the first set after facing two set points in a tiebreaker before Venus broke her sister in the sixth game of the second to go 4-2 up.
Venus fought off a break point in the seventh game to take a 5-2 lead but Serena held in the eighth and faced three set points in the ninth game before reeling off five points in a row to pull back to 5-4.
Serena then evened things up at 5-5 by holding serve only for Venus to edge ahead when she won the 11th game.
Serena faced another set point on her serve but Venus again failed to capitalise. The marathon game saw five deuces before Serena finally put it away to force yet another tiebreaker.
Venus dominated the first three points of the tiebreaker but at 4-2, the sisters produced an incredible point with Serena running all over the court before Venus finally came out on top. Serena fought off three more set points and finally gained an advantage at 8-7 before winning the match when Venus hit a shot long.
"I try not to look at her (Venus) because if I look at her, then I start feeling sorry," Serena added. "I want the best for her. I love her so much. She's my best friend."
The Williams sisters were the only former US Open champions remaining in the field and, with 15 Grand Slam titles between them, there's no doubt that they were the most experienced players left.
Next up for Serena is Dinara Safina, who continued to make her case for the world number one ranking as she reached the semi-finals of the US Open for the first time in her career.
The sixth-seeded Russian made short work of 16th-seeded Flavia Pennetta in the quarter-finals, downing the Italian 6-2 6-3 in 71 minutes.
If the 22-year-old can win the first Grand Slam title of her career at this hardcourt major, she could ascend to number one in the world rankings, joining older brother Marat Safin, who was the number one player in men's tennis earlier in the decade.
Safina, who reached her first Grand Slam final at the French Open, improved to 5-0 lifetime against Pennetta, who was appearing in her first major quarter-final.
"I think maybe I can say finally I'm happy with myself, the way I played, because really I did what I had to do," Safina said. "I was aggressive on the court. I was following the balls every time. Finally I played my game."
The other last-four clash sees number two seed Jelena Jankovic and fifth seed Elena Dementieva face off.
Jankovic will have to lose that match for Safina to have a chance of securing the number one ranking, currently occupied by Serbia's Ana Ivanovic.